Checking a Poisson distribution fit: An example with officer involved shooting deaths WaPo data (R functions)

So besides code on my GitHub page, I have a list of various statistic functions I’ve scripted on the blog over the years on my code snippets page. One of those functions I will illustrate today is some R code to check the fit of the Poisson distribution. Many of my crime analysis examples rely on crime data being approximately Poisson distributed. Additionally it is relevant in regression model building, e.g. should I use a Poisson GLM or do I need to use some type of zero-inflated model?

Here is a brief example to show how my R code works. You can source it directly from my dropbox page. Then I generated 10k simulated rows of Poisson data with a mean of 0.2. So I see many people in CJ make the mistake that, OK my data has 85% zeroes, I need to use some sort of zero-inflated model. If you are working with very small spatial/temporal units of analysis and/or rare crimes, it may be the mean of the distribution is quite low, and so the Poisson distribution is actually quite close.

# My check Poisson function
source('https://dl.dropboxusercontent.com/s/yj7yc07s5fgkirz/CheckPoisson.R?dl=0')

# Example with simulated data
set.seed(10)
lambda <- 0.2
x <- rpois(10000,lambda)
CheckPoisson(x,0,max(x),mean(x))

Here you can see in the generated table from my CheckPoisson function, that with a mean of 0.2, we expect around 81.2% zeroes in the data. And since we simulated the data according to the Poisson distribution, that is what we get. The table shows that out of the 10k simulation rows, 8121 were 0’s, 1692 rows were 1’s etc.

In real life data never exactly conform to hypothetical distributions. But we often want to see how close they are to the hypothetical before building predictive models. A real life example as close to Poisson distributed data as I have ever seen is the Washington Post Fatal Use of Force data. Every year WaPo has been collating the data, the total number of Fatal uses of Police Force in the US have been very close to 1000 events per year. And even in all the turmoil this past year, that is still the case.

# Washington Post Officer Involved Shooting Deaths Data
oid <- read.csv('https://raw.githubusercontent.com/washingtonpost/data-police-shootings/master/fatal-police-shootings-data.csv',
                stringsAsFactors = F)

# Year Stats
oid$year <- as.integer(substr(oid$date,1,4))
year_stats <- table(oid$year)[1:6]
year_stats 
mean(year_stats)
var(year_stats)

One way to check the Poison distribution is that the mean and the variance should be close, and here at the yearly level the data have some evidence of underdispersion according to the Poisson distribution (most crime data is overdispersed – the variance is much greater than the mean). If the actual mean is around 990, you would expect typical variations of say around plus/minus 60 per year (~ 2*sqrt(990)). But that only gives us a few observations to check (6 years). We can dis-aggregate the data to smaller intervals and check the Poisson assumption. Here I aggregate to days (note that this includes zero days in the table levels calculation). Then we again check the fit of the Poisson distribution.

#Now aggregating to count per day
oid$date_val <- as.Date(oid$date)
date_range <- paste0(seq(as.Date('2015-01-01'),max(oid$date_val),by='days'))
day_counts <- as.data.frame(table(factor(oid$date,levels=date_range)))
head(day_counts)
pfit <- CheckPoisson(day_counts$Freq, 0, 10, mean(day_counts$Freq))
pfit

According to the mean and the variance, it appears the distribution is a very close fit to the Poisson. We can see in this data we expected to have around 147 days with 0 fatal encounters, and in reality there were 160. I like seeing the overall counts, but another way is via the proportions in the final three columns of the table. You can see for all of the integers, we are less than 2 percentage points off for any particular integer count. E.g. we expect the distribution to have 3 fatal uses of force on about 22% of the days, but in the observed distribution days with 3 events only happened around 21% of the days (or 20.6378132 without rounding). So overall these fatal use of force data of course are not exactly Poisson distributed, but they are quite close.

So the Poisson distribution is motivated via a process in which the inter-arrival dates of events being counted are independent. Or in more simple terms one event does not cause a future event to come faster or slower. So offhand if you had a hypothesis that publicizing officer fatalities made future officers more hesitant to use deadly force, this is not supported in this data. Given that this is officer involved fatal encounters in the entire US, it is consistent with the data generating process that a fatal encounter in one jurisdiction has little to do with fatal encounters in other jurisdictions.

(Crime data we are often interested in the opposite self-exciting hypothesis, that one event causes another to happen in the near future. Self-excitation would cause an increase in the variance, so the opposite process would result in a reduced variance of the counts. E.g. if you have something that occurs at a regular monthly interval, the counts of that event will be underdispersed according to a Poisson process.)

So the above examples just checked a univariate data source for whether the Poisson distribution was a decent fit. Oftentimes academics are interested in whether the conditional distribution is a good fit post some regression model. So even if the marginal distribution is not Poisson, it may be you can still use a Poisson GLM, generate good predictions, and the conditional model is a good fit for the Poisson distribution. (That being said, you model has to do more work the further away it is from the hypothetical distribution, so if the marginal is very clearly off from Poisson a Poisson GLM probably won’t fit very well.)

My CheckPoisson function allows you to check the fit of a Poisson GLM by piping in varying predicted values over the sample instead of just one. Here is an example where I use a Poisson GLM to generate estimates conditional on the day of the week (just for illustration, I don’t have any obvious reason fatal encounters would occur more or less often during particular days of the week).

#Do example for the day of the week
day_counts$wd <- weekdays(as.Date(day_counts$Var1))
mod <- glm(Freq ~ as.factor(wd) - 1, family="poisson", data=day_counts)
#summary(mod), Tue/Wed/Thu a bit higher
lin_pred <- exp(predict(mod))
pfit_wd <- CheckPoisson(day_counts$Freq, 0, 10, lin_pred)
pfit_wd

You can see that the fit is almost exactly the same as before with the univariate data, so the differences in days of the week does not explain most of the divergence from the hypothetical Poisson distribution, but again this data is already quite close to a Poisson distribution.

So it is common for people to do tests for goodness-of-fit using these tables. I don’t really recommend it – just look at the table and see if it is close. Departures from hypothetical can inform modeling decisions, e.g. if you do have more zeroes than expected than you may need a negative binomial model or a zero-inflated model. If the departures are not dramatic, variance estimates from the Poisson assumption are not likely to be dramatically off-the-mark.

But if you must, here is an example of generating a Chi-Square goodness-of-fit test with the example Poisson fit table.

# If you really want to do a test of fit
chi_stat <- sum((pfit$Freq - pfit$PoisF)^2/pfit$PoisF)
df <- length(pfit$Freq) - 2
dchisq(chi_stat, df)

So you can see in this example the p-value is just under 0.06.

I really don’t recommend this though for two reasons. One is that with null hypothesis significance testing you are really put in a position that large data samples always reject the null, even if the departures are trivial in terms of the assumptions you are making for whatever subsequent model. The flipside of this is that with small samples the test is underpowered, so there are never many good scenarios where it is useful in practice. Two, you can generate superfluous categories (or collapse particular categories) in the Chi-Square test to increase the degrees of freedom and change the p-value.

One of the things though that this is useful for is checking the opposite, people fudging data. If you have data too close to the hypothetical distribution (so very high p-values here), it can be evidence that someone manipulated the data (because real data is never that close to hypothetical distributions). A famous example of this type of test is whether Mendel manipulated his data.

I intentionally chose the WaPo data as it is one of the few that out of the box really appears to be close to Poisson distributed in the wild. One of my next tasks though is to do some similar code for negative binomial fits. Like Paul Allison, for crime count data I rarely see much need for zero-inflated models. But while I was working on that I noticed that the parameters in NB fits with even samples of 1,000 to 10,000 observations were not very good. So I will need to dig into that more as well.

Changepoints in CCTV Effects

So I am a big fan of using splines in regression equations to model non-linear effects. But a limitation of these is that you need to upfront say how many knots you want, as well as where the knots are. So I have explored a bit on fitting models that can identify the changepoints themselves. It was a tricky road, I tried building some in deep learning using pytorch, then tried variational auto-encoders in pyro, then pystan (marginalizing the changepoint out), and then pymc3 (using different samplers). All of my attempts failed! But when I used the R mcp library (Lindeløv, 2020), it was able to find my changepoint using simulated data. (It uses JAGS under the hood, no idea why JAGS behaved better than my other attempts.)

Usecase: Dropoff effect of CCTV on clearance rates

So in spatial criminology, a popular hypothesis is estimating distance decay effects. Ratcliffe (2012) was the first example of using a changepoint regression model to do this, showing a changepoint in the effect of bars on the spatial density of crime nearby. This has been replicated in Xu & Griffiths (2017), and in my work using machine learning and partial dependence plots I show similar changepoint patterns as well (Wheeler & Steenbeek, 2020).

One example use case though I want to mention is not in terms of estimating the spatial density of crime, but with the characteristics of the crime events themselves. Sometimes people I think mistakenly think since I have spatial data, I need to aggregate it to some areal unit, and then do analysis of that areal unit data. That approach is not per-se wrong, but is sometimes a step removed from what you want, and can result in some tricky inferences.

Take for example a recent paper looking at clearances and using RTM by Kennedy et al. (2020). What they do is spatially aggregate homicides cleared and homicides not cleared, and run RTM on each. You might be tempted to interpret if a factor is selected for both models that it does not impact clearances, but it also depends on the size of the effect. So for example, in Brooklyn for drug markets they report a rate ratio of 3.1 and 2.4 (both at the same spatial distance). To translate this into a clearance rate, you need to add the two density estimates for all cases, and then take the cleared cases as the numerator.

# Example R code
clear <- exp(-0.1 + log(3.1))
nonclear <- exp(-0.1 + log(2.4))
prop <- clear/(clear + nonclear)
prop #0.5636364

Here I am treating -0.1 as the intercept. So here this is lower, but close to the overall clearance in Brooklyn, 58%. This 56% will be the estimate iff the intercept for each equation is the same, if they are not though it could change the clearance rate estimate either way. Since the Kennedy paper did not report this, we cannot know. So for instance, if we change the intercept estimates so clearances are higher and non-clearances are lower, we get an estimate that drug markets increase clearances slightly, not decrease them:

clear <- exp(-0.05 + log(3.1))
nonclear <- exp(-0.2 + log(2.4))
prop <- clear/(clear + nonclear)
prop #0.6001124

In this example it probably won’t push them too far either way, but takes a bit of work going from the aggregate data analysis to the estimate we want, how those spatial risk factors impact the clearance rate. There is an easier way though – just incorporate your spatial features, such as the distance the nearest crime generator factor, and estimate a model on the micro level incident data. This is what Kennedy et al. (2020) do later in the paper when incorporating the RTM predictions – I just think they should have done the RTM machinery directly on this problem, instead of the two-step approach.

Examples of my work I have done this approach in the past (incorporating spatial data into the micro level incidents) is with fatalities from gun shot wounds (Circo & Wheeler, 2020). We actually investigated non-linear effects though of distance/drive-time, and did not find evidence of that. Going back to the crime clearance example though, another pre-print I examine the effects of CCTV cameras and find a diminishing effect of case clearances given the distance to the camera (Jung & Wheeler, 2019).

So here we use a pre-post design to show there are some selection effects, and we do further analysis to show this camera bump in clearances is only limited to thefts. But we set the splines at 500, 1000, and 1500 feet pre-emptively for the analysis. A reviewer critique of this is that those three locations are arbitrary (which is correct), so here I will see if I do a changepoint model that allows us to find the knot locations if it will show the same ones.

The idea behind this analysis is that CCTV are often used in investigations. Yeondae is an officer in Korea, same as here in the states first things detectives do is to go and grab CCTV footage. Analysis of cameras are often aggregated to their viewsheds, but I think estimating distance decay effects make as much sense. So events closer to the cameras presumably will provide more clear evidence than events at the border of the viewshed. A second point is that even if the event takes place off-camera, there may be evidence cross by the camera viewshed. Detectives will often try to follow individuals across multiple cameras. So both of those factors suggest a distance decay effect both within a cameras viewshed and a decaying effect even outside of the viewshed. (In addition to this, geo coordinates of crime locations are not perfectly accurate measures either, so that could cause effects outside of the viewshed as well.)

Here I am just limiting the data to the post camera data within 3000 feet for thefts, which still is over 26,000 observations. I’ve posted the data/code to follow along here.

Analysis using mcp in R

Again given my hardship in coding this up myself in python, I created a simulated data example and checked the results using mcp (which you can check in my code). Since mcp recovered my simulated changepoint, (and my python attempts did not), going to go ahead with the mcp library! First, we will import my clearance data and get rid of a few missing cases.

#################
library(mcp)
library(ggplot2)
set.seed(10)
#can see I planned on doing this in pytorch at first!
setwd('D:\\Dropbox\\Dropbox\\Documents\\BLOG\\changepoint_pytorch\\Analysis')
theft_clear <- read.csv('PostTheft_CCTV.csv')
theft_clear <- theft_clear[complete.cases(theft_clear),]
#################

So first for a reference, if I assume there is a linear changepoint at 1000 feet, here are what my results look like. Note here that this is not aggregated data to spatial locations, each row in this dataset is a theft offense, whether it was cleared, and the distance to the nearest CCTV camera.

#################
#What are the coefficients if assume a changepoint of 1000 feet
theft_clear$x_dif <- (theft_clear$CAM.DIST - 1000)*(theft_clear$CAM.DIST > 1000)
theft_mod <- glm(formula = 'STATUSi ~ CAM.DIST + x_dif', family = "binomial", data = theft_clear)
summary(theft_mod) #This gives an estimate of 
#################

And here you can visualize the results alittle easier than trying to back out probabilities for the regression equation:

#################
pred_mod <- predict(theft_mod,type='response')
plot(theft_clear$CAM.DIST,pred_mod, main="Changepoint at 1000 ft",
  xlab="Distance from Camera (ft)", ylab="Probability Clearance")
#################

So this shows clearances nearby cameras in Dallas are around 15%, and they trail off to around 9% at 1000 feet. After that they continue to tail off, but are nearly flat. But again that is assuming a change point at 1000 feet. But the mcp package lets us actual estimate the changepoint itself using Bayesian regression. Here is the set up that is equivalent to my formulation earlier, in that the changepoint cannot be discontinuous.

#################
theft_clear$x <- theft_clear$CAM.DIST 
model = list(
  STATUSi | trials(const) ~ 1 + x,
  ~ 0 + x  #joined changing rate
)

fit = mcp(model, data = theft_clear, family = binomial(), iter = 3000, adapt = 500)
#################

And then if you are following along you can go ahead and take a nap (maybe took 2 hours on my machine?), and when we get back summary(fit) gives us:

So we have very similar coefficients to the manual changepoint model earlier, but the changepoint is around 1600 feet, not 1000. (Although note these are Bayesian credible intervals, not frequentist confidence intervals.) And now to make a nice plot of the fitted model.

#Fitted values for new data
newdat <- data.frame(x = (0:300)*10)
newdat$const <- 1
newdat$CAM.DIST <- newdat$x
res <- fitted(fit, newdata = newdat)

p_pred <- ggplot(data=res) + 
  geom_line(size=1.2, color='black', aes(x = x, y = fitted)) + 
  geom_ribbon(alpha=0.5, fill='black', aes(x = x, ymin=Q2.5 , ymax=Q97.5)) + 
  scale_x_continuous(name="Feet from Camera",breaks=seq(0,3000,500),minor_breaks=NULL) + 
  scale_y_continuous(name="P(Clearance)",breaks=seq(0.06,0.16,0.02),minor_breaks=NULL) +
  theme_bw() + theme(panel.grid.major = element_line(colour = 'grey', linetype = 'dashed', size=0.1)) + 
  theme(text = element_text(size=20))

p_pred

So you can see that here it is a nearly linear drop off until 1600 feet, and then starts to climb back up. The climb up I think is likely due to selection effects, but we can’t 100% rule out displacement effects. Displacement effects could occur with cameras if detectives prioritize events around cameras and de-prioritize other events not nearby cameras. Skeptical that applies to thefts in Dallas though, as they very rarely will be assigned a detective at all.

Wrap Up

So this ended up taking me for a few different turns. One of the things I wanted to be able to test multiple changepoints, maybe if I can ever get pymc3 to give me a reasonable fit, this example is a good illustration. That should also maybe say if you should have no changepoint as well. I think maybe it is much harder to fit those models with binomial data though than with continuous (maybe good for another blog post as well, did simulations at first with 1000 observations and that was a bad idea).

One thing that would be good for evaluating whether change points are reasonable are out of sample predictive comparisons. So say estimate a no changepoint model, a linear changepoint model, and then a model with fixed spline locations. Then see which of those better fits the out of sample data. But since this is a blog post, will leave it as is. But this is a simple illustration to extend prior spatial analysis of changepoints in distance decay effects to one example – crime clearances and CCTV cameras – that I think makes alot of sense.

References

Incorporating treatment non-compliance into call-ins

I have previously published work on identifying optimal individuals to prioritize for call-ins in Focused Deterrence interventions. The idea is we want to identify optimal people to spread the message, so you call in a small number of individuals and they should spread the message to the remaining group. There are better people than others to seed the message to to make sure it spreads throughout the network.

I knew of a direct improvement on that algorithm I published (very similar to the TURF problem I described the other day). But the bigger issue was that even when you call in individuals they do not always come to the meeting – treatment non-compliance. When working with state parole and/or local probation, the police department can ask those agencies to essentially make people come in, but otherwise it is voluntary.

The TURF problem I did the other day gave me a bit of inspiration on how to tackle that treatment non-compliance problem though. In a nutshell when you calculate whether someone is reached (via being directly connected to someone called-in), they can be partially reached based on the probability of the selected nodes treatment compliance. I have posted the code to follow along on dropbox here. I won’t go through the whole thing, but just some highlights.

The Model

First, in some quick and dirty text math, the model is:

Maximize Sum( R_i )

Subject to:

  • R_i <= Sum( S_j*p_j ) for each i
  • Sum( S_j ) = k
  • S_i element of [0,1]
  • R_i <= 1 for each i

Here i refers to an individual node in the gang/group network.

The first constraint R_i <= Sum( S_j*p_j ), the j’s are the nodes that are connected to i (and i itself). The p_j are the estimates that an individual will comply with coming into the call-in. For one agency we worked with for that project, they guessed that those who don’t need to come in comply about 1/6th of the time, so I use that estimate here in my examples, and give people who are on probation/parole a 1 for the probability of compliance.

Second constraint is we can only call in so many people, here k. The model solves very fast, so you can generate results for various k until you get the reach you want to in the end. (You could do the model the other way, minimize S_i while constraining the minimized acceptable reach, e.g. Sum( R_i ) >= threshold, I don’t suggest this in practice though, as when dealing with compliance there may be no feasible solution that gets you the amount of reach in the network you want.)

For the third constraint, the decision variables S_i are binary 0/1’s, but the R_i are continuous. But the trick here is that the last constraint, R_i <= 1, means that the expected reach is capped at 1. Here is a way to think about this, imagine you want to know the chance that person A is reached, and they are connected to two called-in individuals, who each have a 40% chance at complying with the treatment (coming to the call-in). The expected times person A would be reached then is additive in the probabilities, 0.4 + 0.4 = 0.8. If we had 3 people connected to A again at 40% apiece, the expected number of times A would be reached is then 0.4 + 0.4 + 0.4 = 1.2. So a person can be reached multiple times. (Note this is not the probability a person is reached at least once! It is a non-linear problem to model that.)

But if we took away the last constraint, what would happen is that the algorithm would just pick the nodes that had the highest number of neighbors. Since we are maximizing expected reach, if we had a sample of two people, the expected reach values of [2.5, 0] would be preferable to [1, 1], although clearly we rather have the reach spread out. So to prevent that, I cap the expected reach variable at 1, R_i <= 1 for each i, so this spreads out the selected individuals. So in the end the expected number of times people are reached are a lower bound estimate, but those are only people who are expected to receive the message multiple times.

This is a bit of a hack, but in my tests works quite well. I attempted to model the non-linear problem of estimating the probabilities at the person level and still maximizing the expected reach (in the code I have an example of using the CVXR R package). But it was quite fickle in when it would return a solution. So I am focusing on the linear program here, which is not perfect, but is an improvement over my prior published work.

Some Python Snippets

So for my example code, I am using City 4 Gang 4 from my paper. The reason is this was the largest network, and my original algorithm performed the worst. 99 nodes, and my original algorithm identified a 33 person dominant set, but Borgotti’s tool (that uses a genetic algorithm) identified a 29 dominant set.

Here is an example of calling my function to select the individuals for a call-in based on the non-compliance estimates. (g4 is the networkx graph object, the second arg is the number of individuals, and compliance is the node attribute that has the probability of treated compliance.) If we call in only 5 people, we still expect a reach of 29 individuals. Here there ends up being some highly connected people on parole/probation, so they have a 1 probability of complying with the treatment.

A consequence of this algorithm is that if you pipe in 1’s for the treatment compliance, you basically get an improvement to my original algorithm. So for a test we can see if I get the same minimal dominating set as Borgotti did for his algorithm here, where const is just everybody complies 100% of the time.

And yep we get a dominating set (all 99 people are reached). What happens if we go down one, and only select 28 people?

We only reach 98 out of the 99. So it appears a 29 set is the minimal dominating set here. But like I said the treatment non-compliance is a big deal in this setting. What is our expected reach if we take that into account, but still call-in 29 people?

It is still pretty high, around 2/3s of the network, but is still much smaller. Also if you look at the overlap between the constant versus non-compliance model, they select quite a few different individuals. It makes a big difference.

Here is a graph I made of selecting 20 individuals. Red means I selected that person, pink means they are reached at least some, and the size of the reach is proportion to the node. Then grey folks I wouldn’t expect to be reached by the message (at least by first degree connections).

So you can see that most of the people selected have that full 1 expected reach, so the algorithm does prioritize individuals on probation/parole who have a 100% expected compliance. But you can see a few folks who have a lower compliance who are selected as they are in places in the network not covered by those on probation/parole.

I have a tough time getting network layouts to look nice in python (even with the same layout algorithms, I feel like igraph in R just looks much better out of the box).

Future Work

Out of the box, this algorithm could incorporate several different pieces of information. So here I use the non-compliance estimate as a constant, but you could have varying estimates for that based on some other model no problem (e.g. older individuals comply more often than younger, etc.). Also another interesting extension (if you could get estimates) would be the probability a called-in individual spreads the message. In the part Sum( S_j*p_j ) it would just be something like Sum( S_j*p_cj*p_sj ), where p_cj is the compliance probability for attending, and p_sj is the probability to spread the message to those they are connected to.

Getting worthwhile estimates for either of those things will be tough though. Only way I can see it is via some shoe leather qualitative or survey approach.

Simulating runs of events

I still lurk on the Cross Validated statistics site every now and then. There was a kind of common question about the probability of a run of events occurring, and the poster provided a nice analytic solution to the problem using Markov Chains and absorbing states I was not familiar with.

I was familiar with a way to approximate the answer though using a simple simulation, and encoding data via run length encoding. Run length encoding works like this, if you have an original sequence that is AABBBABBBB, then the run length encoded version of this sequence is:

A,2
B,2
A,1
B,4

This is a quite convenient sparse data format to be familiar with. E.g. if you are using tensors in various deep learning libraries, you can encode the data like this and then stack the tensor. But the stacked tensor is just a view, so it doesn’t take up as much memory as the initial full tensor.

Using this encoding also makes a simulation to answer the question, how often do runs of 5+ occur in this hypothetical experiment quite easy to estimate. You just calculate the run length encoded version of the data, and see if any of the lengths are equal to or greater than 5. Below are code snippets in R and Python.

While the analytic solution is of course preferable when you can figure it out, simulations are nice to test whether the solution is correct, as well as to provide an answer when you are not familiar with how to analytically derive a solution.

R Code

R has a native run-length encoding command, rle. The reason is that runs tests are a common time series technique for looking at randomness. Encourage you to run the code yourself to see how my simulated answer lines up with the analytic answer provided on the stats site!

##########################################
# R Code
set.seed(10)
die <- 1:6
run_sim <- function(rolls=1000, conseq=5){
    test <- sample(die,rolls,TRUE)
    res <- max(rle(test)$lengths) >= conseq
    return(res)
}

sims <- 1000000
results <- replicate(sims, run_sim(), TRUE)
print( mean(results) )
##########################################

Python Code

The python code is very similar to the R code. Main difference is there is no native run length encoding command in numpy or scipy I am aware of (although there should be)! So I edited a function I found from Stackoverflow to accomplish the rle.

##########################################
# Python code

import numpy as np
np.random.seed(10)

# Edited from https://stackoverflow.com/a/32681075/604456
# input numpy arrary, return tuple (lengths, vals)
def rle(ia):
    y = np.array(ia[1:] != ia[:-1])         # pairwise unequal (string safe)
    i = np.append(np.where(y), len(ia) - 1) # must include last element
    z = np.diff(np.append(-1, i))           # run lengths
    return (z, ia[i])

die = list(range(6))

def run_sim(rolls=1000, conseq=5):
    rlen, vals = rle(np.random.choice(a=die,size=rolls,replace=True))
    return rlen.max() >= conseq

sims = 1000000
results = [run_sim() for i in range(sims)]
print( sum(results)/len(results) )
##########################################

I debated on expanding this post to show how to do these simulations in parallel, this is a bit of a cheesy experiment to show though. To do 1 million simulations on my machine still only takes like 10~20 seconds for each of these code snippets. So that will have to wait until another post!

You may be thinking why do I care about runs of dice rolls? Well, it can be extended to many different types of time series monitoring problems. For example, when I worked as a crime analyst at Troy I thought about this in terms of analyzing domestic violent reports. They were too numerous for me to read through every report, so I needed to devise a system to identify if there were anomalous patterns in the recent number of reports. You could devise a test here, say how many days of 10+ reports in a row, and see how frequently you would expect that occur in say a year of monitoring. The simulations above could easily be amended to do that, via doing simulations of the Poisson distribution instead of dice rolls, or assigning weights to particular outcomes.

Making aoristic density maps in R

I saw Jerry the other day made/updated an R package to do aoristic analysis. A nice part of this is that it returns the weights breakdown for individual cases, which you can then make maps of. My goto hot spot map for data visualization, kernel density maps, are a bit tough to work with weighted data though in R (tough is maybe not the right word, to use ggplot it takes a bit of work leveraging other packages). So here are some notes on that.

I have provided the data/code here. It is burglaries in Dallas, specifically I filter out just for business burglaries.

R Code Snippet

First, for my front end I load the libraries I will be using, and change the working directory to where my data is located.

############################
library(aoristic) #aoristic analysis 
library(rgdal)    #importing spatial data
library(spatstat) #weighted kde
library(raster)   #manipulate raster object
library(ggplot2)  #for contour graphs
library(sf)       #easier to plot sf objects

my_dir <- "D:\\Dropbox\\Dropbox\\Documents\\BLOG\\aoristic_maps_R\\data_analysis"
setwd(my_dir)
############################

Next I just have one user defined function, this takes an input polygon (the polygon that defines the borders of Dallas here), and returns a raster grid covering the bounding box. It also have an extra data field, to say whether the grid cell is inside/outside of the boundary. (This is mostly convenient when creating an RTM style dataset to make all the features conform to the same grid cells.)

###########################
#Data Manipulation Functions

#B is border, g is size of grid cell on one side
BaseRaster <- function(b,g){
    base_raster <- raster(ext = extent(b), res=g)
    projection(base_raster) <- crs(b)
    mask_raster <- rasterize(b, base_raster, getCover=TRUE) #percentage of cover, 0 is outside
    return(mask_raster)
}
###########################

The next part I grab the datasets I will be using, a boundary file for Dallas (in which I chopped off the Lochs, so will not be doing an analysis of boat house burglaries today), and then the crime data. R I believe you always have to convert date-times when reading from a CSV (it never smartly infers that a column is date/time). And then I do some other data fiddling – Jerry has a nice function to check and make sure the date/times are all in order, and then I get rid of points outside of Dallas using the sp over function. Finally the dataset is for both residential/commercial, but I just look at the commercial burglaries here.

###########################
#Get the datasets

#Geo data
boundary <- readOGR(dsn="Dallas_MainArea_Proj.shp",layer="Dallas_MainArea_Proj")
base_Dallas <- BaseRaster(b=boundary,g=200) 
base_df <- as.data.frame(base_Dallas,long=TRUE,xy=TRUE)

#Crime Data
crime_dat <- read.csv('Burglary_Dallas.csv', stringsAsFactors=FALSE)
#prepping time fields
crime_dat$Beg <- as.POSIXct(crime_dat$StartingDateTime, format="%m/%d/%Y %H:%M:%OS")
crime_dat$End <- as.POSIXct(crime_dat$EndingDateTime, format="%m/%d/%Y %H:%M:%OS")

#cleaning up data
aor_check <- aoristic.datacheck(crime_dat, 'XCoordinate', 'YCoordinate', 'Beg', 'End')
coordinates(crime_dat) <- crime_dat[,c('XCoordinate', 'YCoordinate')]
crs(crime_dat) <- crs(boundary)
over_check <- over(crime_dat, boundary)
keep_rows <- (aor_check$aoristic_datacheck == 0) & (!is.na(over_check$city))
crime_dat_clean <- crime_dat[keep_rows,]

#only look at business burgs to make it go abit faster
busi_burgs <- crime_dat_clean[ crime_dat_clean$UCROffense == 'BURGLARY-BUSINESS', ]
###########################

The next part preps the aoristic weights. First, the aoristic.df function is from Jerry’s aoristic package. It returns the weights broken down by 168 hours per day of the week. Here I then just collapse across the weekdays into the same hour, which to do that is simple, just add up the weights.

After that it is some more geographic data munging using the spatstat package to do the heavy lifting for the weighted kernel density estimate, and then stuffing the result back into another data frame. My bandwidth here, 3000 feet, is a bit large but makes nicer looking maps. If you do this smaller you will have a more bumpy and localized hot spots in the kernel density estimate.

###########################
#aoristic weights

#This takes like a minute
res_weights <- aoristic.df(busi_burgs@data, 'XCoordinate', 'YCoordinate', 'Beg', 'End')

#Binning into same hourly bins
for (i in 1:24){
    cols <- (0:6*24)+i+5
    lab <- paste0("Hour",i)
    res_weights[,c(lab)] <- rowSums(res_weights[,cols])
}

#Prepping the spatstat junk I need
peval <- rasterToPoints(base_Dallas)[,1:2]
spWin <- as.owin(as.data.frame(peval))
sp_ppp <- as.ppp(res_weights[,c('x_lon','y_lat')],W=spWin) #spp point pattern object

#Creating a dataframe with all of the weighted KDE
Hour_Labs <- paste0("Hour",1:24)

for (h in Hour_Labs){
  sp_den <- density.ppp(sp_ppp,weights=res_weights[,c(h)],
                        sigma=3000,
                        edge=FALSE,warnings=FALSE)
  sp_dat <- as.data.frame(sp_den)
  kd_raster <- rasterFromXYZ(sp_dat,res=res(base_Dallas),crs=crs(base_Dallas))
  base_df[,c(h)] <- as.data.frame(kd_raster,long=TRUE)$value
}
###########################

If you are following along, you may be wondering why all the hassle? It is partly because I want to use ggplot to make maps, but for its geom_contour it does not except weights, so I need to do the data manipulation myself to supply ggplot the weighted data in the proper format.

First I turn my Dallas boundary into a simple feature sf object, then I create my filled contour graph, supplying the regular grid X/Y and the Z values for the first Hour of the day (so between midnight and 1 am).

###########################
#now making contour graphs

dallas_sf <- st_as_sf(boundary)

#A plot for one hour of the day
hour1 <- ggplot() + 
  geom_contour_filled(data=base_df, aes(x, y, z = Hour1), bins=9) +
  geom_sf(data=dallas_sf, fill=NA, color='black') +
  scale_fill_brewer(palette="Greens") +
  ggtitle('       Hour [0-1)') + 
  theme_void() + theme(legend.position = "none")
hour1

png('Hour1.png', height=5, width=5, units="in", res=1000, type="cairo") 
hour1
dev.off()
###########################

Nice right! I have in the code my attempt to make a super snazzy small multiple plot, but that was not working out so well for me. But you can then go ahead and make up other slices if you want. Here is an example of taking an extended lunchtime time period.

###########################
#Plot for the afternoon time period
base_df$Afternoon <- rowSums(base_df[,paste0("Hour",10:17)])

afternoon <- ggplot() + 
  geom_contour_filled(data=base_df, aes(x, y, z = Afternoon), bins=9) +
  geom_sf(data=dallas_sf, fill=NA, color='black') +
  scale_fill_brewer(palette="Greens") +
  ggtitle('       Hour [9:00-17:00)') + 
  theme_void() + theme(legend.position = "none")
afternoon
###########################

So you can see that the patterns only slightly changed compared to the midnight prior graph.

Note that these plots will have different breaks, but you could set them to be equal by simply specifying a breaks argument in the geom_contour_filled call.

I will leave it up so someone who is more adept at R code than me to make a cool animated viz over time from this. But that is a way to mash up the temporal weights in a map.

Notes on making Leaflet maps in R

The other day I wrote a blog post for crimrxiv about posting interactive graphics on their pre-print sharing service. I figured it would be good to share my notes on making interactive maps, and to date I’ve mostly created these using the R leaflet library.

The reason I like these interactive maps is they allow you to zoom in and look at hot spots of crime. With the slippy base maps you can then see, oh OK this hot spot is by a train station, or an apartment complex, etc. It also allows you to check out specific data labels via pop-ups as I will show.

I’m using data from my paper on creating cost of crime weighted hot spots in Dallas (that will be forthcoming in Police Quarterly soonish). But I have posted a more direct set of replicating code for the blog post here.

R Code

So first for the R libraries I am using, I also change the working directory to where I have my data located on my Windows machine.

##########################################################
#This code creates a nice leaflet map of my DBSCAN areas

library(rgdal)       #read in shapefiles
library(sp)          #spatial objects
library(leaflet)     #for creating interactive maps
library(htmlwidgets) #for exporting interactive maps

#will need to change baseLoc if replicating on your machine
baseLoc <- "D:\\Dropbox\\Dropbox\\Documents\\BLOG\\leaflet_R_examples\\Analysis"
setwd(baseLoc)
##########################################################

Second, I read in my shapefiles using the rgdal library. This is important, as it includes the projection information. To plot the spatial objects on a slippy map they need to be in the Web Mercator projection (or technically no projection, just a coordinate reference system for the globe). As another trick I like with these basemaps, for the outlined area (the Dallas boundary here), it is easier to plot as a line spatial object, as opposed to an empty filled polygon. You don’t need to worry about the order of the layers as much that way.

##########################################################
#Get the boundary data and DBSCAN data
boundary <- readOGR(dsn="Dallas_MainArea_Proj.shp",layer="Dallas_MainArea_Proj")
dbscan_areas <- readOGR(dsn="db_scan.shp",layer="db_scan")

#Now convert to WGS
DalLatLon <- spTransform(boundary,CRS("+init=epsg:4326"))
DallLine <- as(DalLatLon, 'SpatialLines') #Leaflet useful for boundaries to be lines instead of areas
dbscan_LatLon <- spTransform(dbscan_areas,CRS("+init=epsg:4326") )

#Quick and Dirty plot to check projections are OK
plot(DallLine)
plot(dbscan_LatLon,add=TRUE,col='blue')
##########################################################

Next part, I have a custom function I have made to make pop-up labels for these leaflet maps. First I need to read in a table with the data info for the hot spot areas and merge that into the spatial object. Then the way my custom function works is I pass it the dataset, then I have arguments for the variables I want, and the way I want them labeled. The function does the work of making the labels bolded and putting in line breaks into the HTML. (No doubt others have created nice libraries to do HTML tables/graphs inside the pop-ups that I am unaware of.) If you check out the final print statement, it shows the HTML it built for one of the labels, <strong>ID: </strong>1<br><strong>$ (Thousands): </strong>116.9<br><strong>PAI: </strong>10.3<br><strong>Street Length (Miles): </strong>0.4

##########################################################
#Function for labels

#read in data
crime_stats <- read.csv('ClusterStats_wlen.csv', stringsAsFactors=FALSE)
dbscan_stats <- crime_stats[crime_stats$type == 'DBSCAN',]
dbscan_stats$clus_id <- as.numeric(dbscan_stats$AreaStr) #because factors=False!

#merge into the dbscan areas
dbscan_LL <- merge(dbscan_LatLon,dbscan_stats)

LabFunct <- function(data,vars,labs){
  n <- length(labs)
  add_lab <- paste0("<strong>",labs[1],"</strong>",data[,vars[1]])
  for (i in 2:n){
    add_lab <- paste0(add_lab,"<br><strong>",labs[i],"</strong>",data[,vars[i]])
  }
  return(add_lab)
}

#create labels
vs <- c('AreaStr', 'val_th', 'PAI_valth_len', 'LenMile')
#Lazy, so just going to round these values
for (v in vs[-1]){
  dbscan_LL@data[,v] <- round(dbscan_LL@data[,v],1)
}  
lb <- c('ID: ','$ (Thousands): ','PAI: ','Street Length (Miles): ')
diss_lab <- LabFunct(dbscan_LL@data, vs, lb)

print(diss_lab[1]) #showing off just one
##########################################################

Now finally onto the hotspot map. This is a bit to chew over, so I will go through bit-by-bit.

##########################################################
HotSpotMap <- leaflet() %>%
  addProviderTiles(providers$OpenStreetMap, group = "Open Street Map") %>%
  addProviderTiles(providers$CartoDB.Positron, group = "CartoDB Lite") %>%
  addPolylines(data=DallLine, color='black', weight=4, group="Dallas Boundary") %>%
  addPolygons(data=dbscan_LL,color = "blue", weight = 2, opacity = 1.0, 
              fillOpacity = 0.5, group="DBSCAN Areas",popup=diss_lab, 
              highlight = highlightOptions(weight = 5,bringToFront = TRUE)) %>%
  addLayersControl(baseGroups = c("Open Street Map","CartoDB Lite"),
                   overlayGroups = c("Dallas Boundary","DBSCAN Areas"),
                   options = layersControlOptions(collapsed = FALSE))  %>%
  addScaleBar(position = "bottomleft", options = scaleBarOptions(maxWidth = 100, 
              imperial = TRUE, updateWhenIdle = TRUE))
                      
HotSpotMap #this lets you view interactively

#or save to a HTML file to embed in webpage
saveWidget(HotSpotMap,"HotSpotMap.html", selfcontained = TRUE)
##########################################################

First I create the empty leaflet() object. Because I am superimposing multiple spatial layers, I don’t worry about setting the default spatial layer. Second, I add in two basemap providers, OpenStreetMap and the grey scale CartoDB positron. Positron is better IMO for visualizing global data patterns, but the open street map is better for when you zoom in and want to see exactly what is around a hot spot area. Note when adding in a layer, I give it a group name. This allows you to later toggle which provider you want via a basegroup in the layers control.

Next I add in the two spatial layers, the Dallas Boundary lines and then the hot spots. For the DBSCAN hot spots, I include a pop-up diss_lab for the dbscan hot spot layer. This allows you to click on the polygon, and you get the info I stuffed into that label vector earlier. The HTML is to make it print nicely.

Finally then I add in a layers control, so you can toggle layers on/off. Basegroups mean that only one of the options can be selected, it doesn’t make sense to have multiple basemaps selected. Overlay you can toggle on/off as needed. Here the overlay doesn’t matter much due to the nature of the map, but if you have many layers (e.g. a hot spot map and a choropleth map of demographics) being able to toggle the layers on/off helps a bit more.

Then as a final touch I add in a scale bar (that automatically updates depending on the zoom level). These aren’t my favorite with slippy maps, as I’m not even 100% sure what location the scale bar refers to offhand (the center of the map? Or literally where the scale bar is located?) But when zoomed into smaller areas like a city I guess it is not misleading.

Here is a screenshot of this created map zoomed out to the whole city using the Positron grey scale base map. So it is tough to visualize the distribution of hot spots from this. If I wanted to do that in a static map I would likely just plot the hot spot centroids, and then make the circles bigger for areas that capture more crime.

But since we can zoom in, here is another screenshot zoomed in using the OpenStreetMap basemap, and also illustrating what my pop-up labels look like.

I’m too lazy to post this exact map, but it is very similar to one I posted for my actual hot spots paper if you want to check it out directly. I host it on GitHub for free.

Here I did not show how to make a choropleth map, but Jacob Kaplan in his R book has a nice example of that. And in the future I will have to update this to show how to do the same thing in python using the Folium library. I used Folium in this blog post if you want to dig into an example though for now.

Some more examples

For some other examples of what is possible in Leaflet maps in R, here are some examples I made for my undergrad Communities and Crime class. I had students submit prediction assignments (e.g. predict the neighborhood with the most crime in Dallas, predict the street segment in Oak Cliff with the most violent crime, predict the bar with the most crimes nearby, etc.) I would then show the class the results, as well as where other students predicted. So here are some screen shots of those maps.

Choropleth

Graduated Points

Street Segment Viz

Some additional plots to go with Crime Increase Dispersion

So Jerry nerdsniped me again with his Crime Increase Dispersion statistic (Ratcliffe, 2010). Main motivation for this post is that I don’t find that stat very intuitive to be frank. So here are some alternate plots, based on how counts of crime approximately follow a Poisson distribution. These get at the same question though as Jerry’s work, is a crime increase (or decrease) uniform across the city or specific to a few particular sub-areas.

First, in R I am going to simulate some data. This creates a set of data that has a constant increase over 50 areas of 20%, but does the post crime counts as Poisson distributed (so it isn’t always exactly a 20% increase). I then create 3 outliers (two low places and one high place).

###########################################
#Setting up the simulation
set.seed(10)
n <- 50
low <- 10
hig <- 400
inc <- 0.2
c1 <- trunc(runif(n,low,hig))
c2 <- rpois(n,(1+inc)*c1)
#Putting in 2 low outliers and 1 high outlier
c2[5] <- c1[5]*0.5
c2[10] <- c1[10]*0.5
c2[40] <- c1[40]*2
#data frame for ggplot
my_dat <- data.frame(pre=c1,post=c2)
###########################################

The first plot I suggest is a simple scatterplot of the pre-crime counts on the X axis vs the post-crime counts on the Y axis. My make_cont function takes those pre and post crime counts as arguments and creates a set of contour lines to put as a backdrop to the plot. Points within those lines support the hypothesis that the area increased in crime at the same rate as the overall crime increase, taking into account the usual ups and downs you would expect with Poisson data. This is very similar to mine and Jerry’s weighted displacement difference test (Wheeler & Ratcliffe, 2018), and uses a normal based approximation to examine the differences in Poisson data. I default to plus/minus three because crime data tends to be slightly over-dispersed (Wheeler, 2016), so coverage with real data should be alittle better (although here is not necessary).

###########################################
#Scatterplot of pre vs post with uniform 
#increase contours

make_cont <- function(pre_crime,post_crime,levels=c(-3,0,3),lr=10,hr=max(pre_crime)*1.05,steps=1000){
    #calculating the overall crime increase
    ov_inc <- sum(post_crime)/sum(pre_crime)
    #Making the sequence on the square root scale
    gr <- seq(sqrt(lr),sqrt(hr),length.out=steps)^2
    cont_data <- expand.grid(gr,levels)
    names(cont_data) <- c('x','levels')
    cont_data$inc <- cont_data$x*ov_inc
    cont_data$lines <- cont_data$inc + cont_data$levels*sqrt(cont_data$inc)
    return(as.data.frame(cont_data))
}

contours <- make_cont(c1,c2)

library(ggplot2)
eq_plot <- ggplot() + 
           geom_line(data=contours, color="darkgrey", linetype=2, 
                     aes(x=x,y=lines,group=levels)) +
           geom_point(data=my_dat, shape = 21, colour = "black", fill = "grey", size=2.5, 
                      alpha=0.8, aes(x=pre,y=post)) +
           scale_y_continuous(breaks=seq(0,500,by=100)) +
           coord_fixed() +
           xlab("Pre Crime Counts") + ylab("Post Crime Counts")
           #scale_y_sqrt() + scale_x_sqrt() #not crazy to want square root scale here
eq_plot

#weighted correlation to view the overall change
cov.wt(my_dat[,c('pre','post')], wt = 1/sqrt(my_dat$pre), cor = TRUE)$cor[1,2]
########################################### 

So places that are way outside the norm here should pop out, either for increases or decreases. This will be better than Jerry’s stats for identifying outliers in lower baseline crime places.

I also show how to get an overall index based on a weighted correlation coefficient on the last line (as is can technically return a value within (-1,1), so might square it for a value within (0,1)). But I don’t think the overall metric is very useful – it has no operational utility for a crime department deciding on a strategy. You always need to look at the individual locations, no matter what the overall index metric says. So I think you should just cut out the middle man and go straight to these plots. I’ve had functionally similar discussions with folks about Martin Andresen’s S index metric (Wheeler, Steenbeek, Andresen, 2018), just make your graphs and maps!

An additional plot that basically takes the above scatterplot and turns it on its side is a Poisson version of a Bland-Altman plot. Traditionally this plot is the differences of two measures on the Y axis, and the average of the two measures on the X axis. Here to make the measures have the same variance, I divide the post-pre crime count differences by sqrt(post+pre). This is then like a Poison Z-score, taking into account the null of an equal increase (or decrease) in crime stats among all of the sub-areas. (Here you might also use the Poisson e-test to calculate p-values of the differences, but the normal based approximation works really well for say crime counts of 5+.)

###########################################
#A take on the Bland-Altman plot for Poisson data

ov_total <- sum(my_dat$post)/sum(my_dat$pre)
my_dat$dif <- (my_dat$post - ov_total*my_dat$pre)/sqrt(my_dat$post + my_dat$pre)
my_dat$ave <- (my_dat$post + my_dat$pre)/2

ba_plot <- ggplot(data=my_dat, aes(x=ave, y=dif)) + 
           geom_point(shape = 21, colour = "black", fill = "grey", size=2.5, alpha=0.8) +
           scale_y_continuous(breaks=seq(-8,6,by=2)) +
           xlab("Average Crime") + ylab("Z-score (Equal Increase)")

ba_plot

#false discovery rate correction
my_dat$p_val <- pnorm(-abs(my_dat$dif))*2 #two-tailed p-value
my_dat$p_adj <- p.adjust(my_dat$p_val,method="BY") #BY correction since can be correlated
my_dat <- my_dat[order(my_dat$p_adj),]
my_dat #picks out the 3 cases I adjusted
###########################################

So again places with large changes that do not follow the overall trend will pop out here, both for small and large crime count places. I also show here how to do a false-discovery rate correction (same as in Wheeler, Steenbeek, & Andresen, 2018) if you want to actually flag specific locations for further investigation. And if you run this code you will see it picks out my three outliers in the simulation, and all other adjusted p-values are 1.

One thing to note about these tests are they are conditional on the observed overall citywide crime increase. If it does happen that only one area increased by alot, it may make more sense to set these hypothesis tests to a null of equal over time. If you see that one area is way above the line and a ton are below the line, this would indicate that scenario. To set the null to no change in these graphs, for the first one just pass in the same pre estimates for both the pre and post arguments in the make_cont function. For the second graph, change ov_total <- 1 would do it.

References

  • Ratcliffe, J. H. (2010). The spatial dependency of crime increase dispersion. Security Journal, 23(1), 18-36.
  • Wheeler, A. P. (2016). Tables and graphs for monitoring temporal crime trends: Translating theory into practical crime analysis advice. International Journal of Police Science & Management, 18(3), 159-172.
  • Wheeler, A. P., & Ratcliffe, J. H. (2018). A simple weighted displacement difference test to evaluate place based crime interventions. Crime Science, 7(1), 11.
  • Wheeler, A. P., Steenbeek, W., & Andresen, M. A. (2018). Testing for similarity in area‐based spatial patterns: Alternative methods to Andresen’s spatial point pattern test. Transactions in GIS, 22(3), 760-774.

Using regularization to generate synthetic controls and conformal prediction for significance tests

When viewing past synthetic control results, one of things that has struck me is that the matching of the pre-trends is really good — almost too good in many cases (appears to be fitting to noise, although you may argue that is a feature in terms of matching exogenous shocks). For example, if you end up having a pre-treatment series of 10 years, and you have a potential donor pool the size of 30, you could technically pick 10 of them at random, fit a linear regression predicting the 10 observations in the treated unit, based on 10 covariates of the donor pool outcomes over the same pre time period, and get perfect predictions (ignoring the typical constraints one places on the coefficients).

So how do we solve that problem? One solution is to use regularized regression results (e.g. ridge regression, lasso), when the number of predictors is greater than the number of observations. So I can cast the matching procedure into a regression problem to generate the weights. Those regression procedures are typically used for forecasting, but don’t have well defined standard errors, and so subsequently are typically only used for point forecasts. One way to make inferences though is to generate the synthetic weights (here using lasso regression), and then use conformal prediction intervals to do our hypothesis testing of counterfactual trends.

Here I walk through an example using state panel crime data in R, full code and data can be downloaded here.

A Synthetic Control Example

So first, these are the packages we need to replicate the results. conformalInference is not on CRAN yet, so use devtools to install it.

#library(devtools)
#install_github(repo="ryantibs/conformal", subdir="conformalInference")
library(conformalInference)
library(glmnet)
library(Synth)

Then I have prepped a nice state panel dataset of crime rates and counts from 1960 through 2014. I set a hypothetical treatment start year in 2005 just so I have a nice 10 years post data for illustration. That is a pretty good length pre-panel though, and a good number of potential donors.

MyDir <- "C:\\Users\\axw161530\\Desktop\\SynthIdeas"
setwd(MyDir)

TreatYear <- 2005

LongData <- read.csv("CrimeStatebyState_Edited.csv")
summary(LongData)

Next I prep my data, currently it is in long panel format, but I need it in wide format to fit the regression equations I want. I am just matching on violent crime rates here. I take out NY, as it is missing a few years of data. (This dataset also includes DC.) Then I split it up into my pre intervention and post intervention set.

#Changing the data to wide for just the violent offenses
wide <- LongData[,c('State','Year','Violent.Crime.rate')]
names(wide)[3] <- 'VCR'
wide <- reshape(wide, idvar="Year", timevar="State", direction="wide")
summary(wide)
#Take out NY because of NAs
wide <- wide[,c(1:33,35:52)]

wide_pre <- as.matrix(wide[wide$Year < TreatYear,])
wide_post <- as.matrix(wide[wide$Year >= TreatYear,])

Now onto the good stuff, we can estimate our lasso regression using the pre-data to get our weights. This constrains the coefficients to be positive and below 1. But does not have the constraint they sum to 1. I just choose Alabama as an example treated unit — I intentionally chose a state and year that should not have any effects for illustration and to check the coverage of my technique vs more traditional analyses.

You can see in my notes this is different than traditional synth in that it has an intercept as well. I was surprised, but the predictions in sample were really bad without the intercept no matter how I sliced it.

res <- glmnet(x=wide_pre[,3:51],y=wide_pre[,2],family="gaussian",
       lower.limits=0,upper.limits=1,intercept=TRUE,standardize=FALSE,
       alpha=1) #need the intercept, predictions suck otherwise

Even though this does not constrain the coefficients to sum to 1, it ends up with weights really close to that ideal anyway (sum of the non-intercept coefficients is just over 1.01). When I use crossvalidation it does not choose weights that sum to unity, but in sample the above code and the cv.glmnet are really similar in terms of predictions.

co_ridge <- as.matrix(coef(res))
fin <- co_ridge[,"s99"]
active <- fin[fin > 0] #Does not include intercept

If you print active we then have for our state weights (and the intercept is pretty tiny, -22). So not quite sure why eliminating the intercept was causing such problems in this example. So North Carolina just sneaks in, but otherwise the synthetic control is a mix of Arkansas, California, Kentucky, and Texas. The intercept is just a level shift, so we are still matching curves otherwise, so that does not bother me very much.

VCR.AR 0.2078156362
VCR.CA 0.1201658279
VCR.IL 0.1543015666
VCR.KY 0.2483613907
VCR.NC 0.0002896238
VCR.TX 0.2818272850

If we look at our predictions for the pre-time period, Alabama had the typical crime path, with a big raise going into the early 90’s and then a fall afterward (black line), and our in-sample predictions from the lasso regression are decent.

pre_pred <- predict(res,newx=wide_pre[,3:51],s=min(res$lambda)) #for not cv results

plot(wide_pre[,1],wide_pre[,2],type='l',xlab='',ylab='Violent Crime Rate per 100,000')
points(wide_pre[,1],pre_pred,bg='red',pch=21) #Not too shabby
legend(1960,800,legend=c("Observed Albama","Predicted"),col=c("black","black"), pt.bg=c("black","red"), lty=c(1,NA), pch=c(NA,21))

Now to evaluate post intervention, we are going to generate conformal prediction intervals using a jackknife approach. Basically doing all the jazz of above, but leaving one pre year out at a time, and trying to predict Alabama’s violent crime rate for that left out year. Repeat that same process for all prior years, and we can get a calculation of the standard error of our prediction. Then apply that standard error to future years, so we can tell if the observed trend is different than the counterfactual we estimated (given the counterfactual has errors). I generate both 90% prediction intervals, as well as 99% prediction intervals.

train_fun <- function(x, y, out=NULL){
  return( glmnet(x,y,alpha=1,standardize=FALSE,intercept=TRUE,nlambda=100,
                lower.limits=0,upper.limits=1,family="gaussian")
  )
}

pred_fun = function(out, newx) {
    return(predict(out, newx, s=min(out$lambda)))
}

limits_10 <- conformal.pred.jack(x=wide_pre[,3:51],y=wide_pre[,2],x0=wide_post[,3:51],
                                 train.fun=train_fun,predict.fun=pred_fun,alpha=0.10,
                                 verbose=TRUE)

limits_01 <- conformal.pred.jack(x=wide_pre[,3:51],y=wide_pre[,2],x0=wide_post[,3:51],
               train.fun=train_fun,predict.fun=pred_fun,alpha=0.01,
               verbose=TRUE)

plot(wide_post[,1],wide_post[,2],type='l',ylim=c(150,650),xlab='',ylab='Violent Crime Rate per 100,000')
points(wide_post[,1],post_pred,bg='red',pch=21)
lines(wide_post[,1],limits_10$lo,col='grey')
lines(wide_post[,1],limits_10$up,col='grey')
lines(wide_post[,1],limits_01$lo,col='grey',lwd=3)
lines(wide_post[,1],limits_01$up,col='grey',lwd=3)
legend("topright",legend=c("Observed Albama","Predicted","90% Pred. Int.","99% Pred. Int."),cex=0.7,
       col=c("black","black","grey","grey"), pt.bg="red", lty=c(1,NA,1,1), pch=c(NA,21,NA,NA), lwd=c(1,1,1,3))

Then at the end of the above code snippet I made a plot. Black line is observed for Alabama from 05-14. Red dots are the estimated counterfactual based on the pre-weights. The lighter grey lines are then the prediction intervals. So we can see it is just outside the 90% intervals 3 times in the later years (would only expect 1 time), but all easily within the 99% intervals.

Note these are prediction intervals, not confidence intervals. Thinking about it I honestly don’t know whether we want prediction or confidence intervals in this circumstance, but prediction will be wider.

So this approach just matches on the pre-treated same outcome observations. To match on additional covariates, you can add them in as rows into the pre-treatment dataset (although you would want to normalize the values to a similar mean and standard deviation as the pre-treated outcome series).

You may also add in other covariates, like functions of time (although this changes the nature of the identification). So for example say you incorporate a linear and quadratic trend in time, and lasso only chooses those two time factors and no control areas. You are doing something more akin to interrupted time series analysis at that point (the counterfactual is simply based on your estimate of the pre-trend). Which I think is OK sometimes, but is quite different than using control areas to hopefully capture random shocks.

Comparing to Traditional Synth results

To see whether my error intervals are similar to the placebo approach, I used the old school synth R package. It isn’t 100% comparable, as it makes you match on at least one covariate, so here I choose to also match on the average logged population over the pre-treatment period.

#NY is missing years
LongData_MinNY <- LongData[as.character(LongData$State) != "NY",c("State","Year","Violent.Crime.rate","Population")]
LongData_MinNY$StateNum <- as.numeric(LongData_MinNY$State)
LongData_MinNY$State <- as.character(LongData_MinNY$State)
LongData_MinNY$LogPop <- log(LongData_MinNY$Population)    

state_nums <- unique(LongData_MinNY$StateNum)
    
dataprep.out <- dataprep(foo = LongData_MinNY,
                         dependent = "Violent.Crime.rate",
                         predictors = c("LogPop"),
                         unit.variable = "StateNum",
                         unit.names.variable = "State",
                         time.variable = "Year",
                         treatment.identifier = 2,
                         controls.identifier = state_nums[!state_nums %in% 2],
                         time.optimize.ssr = 1960:(TreatYear-1),
                         time.predictors.prior = 1960:(TreatYear-1),
                         time.plot = 1960:2014
                         )

synth_res <- synth(dataprep.out)
synth_tables <- synth.tab(dataprep.res = dataprep.out, synth.res = synth_res)
synth_tables$tab.w #a bunch of little weights across the board
path.plot(synth.res = synth_res, dataprep.res = dataprep.out, tr.intake=TreatYear,Xlab='',Ylab='Violent Crime Rate per 100,000',
      Legend=c("Alabama","Synthetic Control"), Legend.position=c("topleft"))

Looking at the weights, it is a bunch of little ones for many different states. Looking at the plot, it doesn’t appear to be any better fit than the lasso approach.

And then I just do the typical approach and use placebo checks to do inference. I loop over my 49 placebos (-1 state for NY, but +1 state because this list includes DC).

#Dataframes to stuff the placebos check results into
Predicted <- data.frame(dataprep.out$Y0plot %*% synth_res$solution.w)
names(Predicted) <- "TreatPred"

Pred_MinTreat <- data.frame(TreatPred = Predicted$TreatPred - LongData_MinNY[LongData_MinNY$StateNum == 2,"Violent.Crime.rate"])

#Now I just need to loop over the other states and collect their results for the placebo tests

placebos <- state_nums[!state_nums %in% 2]
for (i in placebos){
  dataprep.plac <- dataprep(foo = LongData_MinNY,
                           dependent = "Violent.Crime.rate",
                           predictors = c("LogPop"),
                           unit.variable = "StateNum",
                           unit.names.variable = "State",
                           time.variable = "Year",
                           treatment.identifier = i,
                           controls.identifier = state_nums[!state_nums %in% i],
                           time.optimize.ssr = 1960:(TreatYear-1),
                           time.predictors.prior = 1960:(TreatYear-1),
                           time.plot = 1960:2014
  )
  synth_resP <- synth(dataprep.plac)
  synth_tablesP <- synth.tab(dataprep.res = dataprep.plac, synth.res = synth_resP)
  nm <- paste0("S.",i)
  Predicted[,nm] <- dataprep.plac$Y0plot %*% synth_resP$solution.w
  Pred_MinTreat[,nm] <- Predicted[,nm] - LongData_MinNY[LongData_MinNY$StateNum == i,"Violent.Crime.rate"]
}

If you look at the synth estimates for Alabama (grey circles), they are almost exactly the same as the lasso predictions (red circles), even though the weights are very different.

PredRecent <- Predicted[1960:2014 >= TreatYear,]
DiffRecent <- Pred_MinTreat[1960:2014 >= TreatYear,]

plot(wide_post[,1],wide_post[,2],type='l',ylim=c(100,700),xlab='',ylab='Violent Crime Rate per 100,000')
points(wide_post[,1],post_pred,bg='red',pch=21)
lines(wide_post[,1],limits_10$lo,col='grey')
lines(wide_post[,1],limits_10$up,col='grey')
lines(wide_post[,1],limits_01$lo,col='grey',lwd=3)
lines(wide_post[,1],limits_01$up,col='grey',lwd=3)
points(wide_post[,1],PredRecent$TreatPred,bg='grey',pch=21)
legend("topright",legend=c("Observed Albama","Lasso Pred.","90% Pred. Int.","99% Pred. Int.","Synth Pred."),cex=0.6,
       col=c("black","black","grey","grey"), pt.bg=c(NA,"red",NA,NA,"grey"), lty=c(1,NA,1,1,NA), pch=c(NA,21,NA,NA,21), lwd=c(1,1,1,3,1))

But when we look at variation in our placebo results (thin, purple lines), they are much wider than our conformal prediction intervals.

plot(wide_post[,1],wide_post[,2]-post_pred,type='l',ylim=c(-500,500),xlab='',ylab='Observed - Predicted (Violent Crime Rates)')
points(wide_post[,1],post_pred-post_pred,bg='red',pch=21)
lines(wide_post[,1],limits_01$lo-post_pred,col='grey',lwd=3)
lines(wide_post[,1],limits_01$up-post_pred,col='grey',lwd=3)

for (i in 2:ncol(PredRecent)){
  lines(wide_post[,1],DiffRecent[,i],col='#9400D340',lwd=0.5)
}

legend(x=2005.5,y=-700,legend=c("Observed Albama","Lasso Pred.","99% Pred. Int.","Placebos"),
       col=c("black","black","grey",'#9400D3'), pt.bg=c(NA,"red",NA,NA), lty=c(1,NA,1,1), 
       pch=c(NA,21,NA,NA), lwd=c(1,1,3,0.5), xpd=TRUE, horiz=TRUE, cex = 0.45)

So I was hoping they would be the same (conformal would cover the placebo at the expected rate), but alas they are not. So I’m not sure if my conformal intervals are too small, or the placebo checks are extra noisy. I can’t prove it, but I suspect the placebo checks are somewhat noisy, mainly because there will always be some intervention that is idiosyncratic to specific donors over long periods of time that makes them no longer good counterfactuals. This seems especially true if you consider predictions further out from the treatment year. Although I find the logic of the placebo checks pretty convincing, so I am somewhat torn.

Since we have in this example 49 donors, the two-tailed p-value for being outside the placebos would be 2/(49+1)=0.04. Here we would need an intervention that either increased violent crime rates by plus/minus 400 per 100,000, pretty much an impossible standard given a baseline of only 400 crimes per 100,000 as of 2004. The 99% conformal intervals are still pretty wide, with an increase/decrease of about 150 violent crimes per 100,000 needed to be a significant change. The two lines way outside 400 happen to be Alaska and Wyoming, not DC, so maybe a tiny population state results in higher volatility problem. But besides them there are a bunch of placebo states around plus/minus 300 as well.

So caveat emptor if you want to use this idea in your own work, I don’t know if my suggestion is good or bad. Here it suggests its more diagnostic (smaller intervals) than the placebo checks, and isn’t limited by the number of potential donors in setting the alpha level for your tests (e.g. if you only have 10 potential donors your placebo checks are only 90% intervals).

Since this is just one example, there are a few things I would need to know before recommending it more generally. One is that it may not work with smaller pre time series and/or a smaller donor pool. (Not sure of any better way of checking than via a ton of different simulations.)

More general notes

Doing some more lit review while preparing this post, I appear to be like 15th in line to suggest this approach (so don’t take it as novel). In terms of using the lasso to estimate the synth weights, it seems Susan Athey and colleagues proposed something similar in addition to using other machine learning techniques. Also see Amjad et al. 2018 in the Journal of Machine Learning, and this workshop by Alex Hollingsworth and Coady Wing. I am not even the first one to think to use conformal prediction intervals apparently, see this working paper (Chernozhukov, Wuthrich, and Zhu, 2019) posted just a few weeks prior.

There is another R package, gsynth, that appears to solve the problem of p > n via a variable reduction technique (Xu, 2017). Xu also discusses how incorporating more information is really making different identification assumptions. So again just getting good predictions/minimizing the in-sample mean square error is not necessarily the right approach to get correct causal inferences.

Just a blog post, so again can’t say if this is an improvement over other work offhand. This is just illustrative that the bounds for the conformal prediction may be smaller than the typical permutation based approach. Casting it as a regression problem I intuitively grok more, and think opens up more possibilities. For example, you may want to use binomial logistic models instead of linear for the fitting process (so takes into account more volatility for smaller population states).

 

Making a hexbin map in ggplot

In a recent working paper I made a hexbin map all in R. (Gio did most of the hard work of data munging and modeling though!) Figured I would detail the process here for some notes. Hexagon binning is purportedly better than regular squares (to avoid artifacts of runs in discretized data). But the reason I use them in this circumstance is mostly just an aesthetic preference.

Two tricky parts to this: 1) making the north arrow and scale bar, and 2) figuring out the dimensions to make regular hexagons. As an illustration I use the shooting victim data from Philly (see the working paper for all the details) full data and code to replicate here. I will walk through a bit of it though.

Data Prep

First to start out, I just use these three libraries, and set the working directory to where my data is.

library(ggplot2)
library(rgdal)
library(proj4)
setwd('C:\\Users\\axw161530\\Dropbox\\Documents\\BLOG\\HexagonMap_ggplot\\Analysis')

Now I read in the Philly shooting data, and then an outline of the city that is projected. Note I read in the shapefile data using rgdal, which imports the projection info. I need that to be able to convert the latitude/longitude spherical coordinates in the shooting data to a local projection. (Unless you are making a webmap, you pretty much always want to use some type of local projection, and not spherical coordinates.)

#Read in the shooting data
shoot <- read.csv('shootings.csv')
#Get rid of missing
shoot <- shoot[!is.na(shoot$lng),c('lng','lat')]
#Read in the Philly outline
PhilBound <- readOGR(dsn="City_Limits_Proj.shp",layer="City_Limits_Proj")
#Project the Shooting data
phill_pj <- proj4string(PhilBound)
XYMeters <- proj4::project(as.matrix(shoot[,c('lng','lat')]), proj=phill_pj)
shoot$x <- XYMeters[,1]
shoot$y <- XYMeters[,2]

Making a Basemap

It is a bit of work to make a nice basemap in R and ggplot, but once that upfront work is done then it is really easy to make more maps. To start, the GISTools package has a set of functions to get a north arrow and scale bar, but I have had trouble with them. The ggsn package imports the north arrow as a bitmap instead of vector, and I also had a difficult time with its scale bar function. (I have not figured out the cartography package either, I can’t keep up with all the mapping stuff in R!) So long story short, this is my solution to adding a north arrow and scale bar, but I admit better solutions probably exist.

So basically I just build my own polygons and labels to add into the map where I want. Code is motivated based on the functions in GISTools.

#creating north arrow and scale bar, motivation from GISTools package
arrow_data <- function(xb, yb, len) {
  s <- len
  arrow.x = c(0,0.5,1,0.5,0) - 0.5
  arrow.y = c(0,1.7  ,0,0.5,0)
  adata <- data.frame(aX = xb + arrow.x * s, aY = yb + arrow.y * s)
  return(adata)
}

scale_data <- function(llx,lly,len,height){
  box1 <- data.frame(x = c(llx,llx+len,llx+len,llx,llx),
                     y = c(lly,lly,lly+height,lly+height,lly))
  box2 <- data.frame(x = c(llx-len,llx,llx,llx-len,llx-len),
                     y = c(lly,lly,lly+height,lly+height,lly))
  return(list(box1,box2))
}

x_cent <- 830000
len_bar <- 3000
offset_scaleNum <- 64300
arrow <- arrow_data(xb=x_cent,yb=67300,len=2500)
scale_bxs <- scale_data(llx=x_cent,lly=65000,len=len_bar,height=750)

lab_data <- data.frame(x=c(x_cent, x_cent-len_bar, x_cent, x_cent+len_bar, x_cent),
                       y=c( 72300, offset_scaleNum, offset_scaleNum, offset_scaleNum, 66500),
                       lab=c("N","0","3","6","Kilometers"))

This is about the best I have been able to automate the creation of the north arrow and scale bar polygons, while still having flexibility where to place the labels. But now we have all of the ingredients necessary to make our basemap. Make sure to use coord_fixed() for maps! Also for background maps I typically like making the outline thicker, and then have borders for smaller polygons lighter and thinner to create a hierarchy. (If you don’t want the background map to have any color, use fill=NA.)

base_map <- ggplot() + 
            geom_polygon(data=PhilBound,size=1.5,color='black', fill='darkgrey', aes(x=long,y=lat)) +
            geom_polygon(data=arrow, fill='black', aes(x=aX, y=aY)) +
            geom_polygon(data=scale_bxs[[1]], fill='grey', color='black', aes(x=x, y = y)) + 
            geom_polygon(data=scale_bxs[[2]], fill='white', color='black', aes(x=x, y = y)) + 
            geom_text(data=lab_data, size=4, aes(x=x,y=y,label=lab)) +
            coord_fixed() + theme_void()

#Check it out           
base_map

This is what it looks like on my windows machine in RStudio — it ends up looking alittle different when I export the figure straight to PNG though. Will get to that in a minute.

Making a hexagon map

Now you have your basemap you can superimpose whatever other data you want. Here I wanted to visualize the spatial distribution of shootings in Philly. One option is a kernel density map. I tend to like aggregated count maps though better for an overview, since I don’t care so much for drilling down and identifying very specific hot spots. And the counts are easier to understand than densities.

In geom_hex you can supply a vertical and horizontal parameter to control the size of the hexagon — supplying the same for each does not create a regular hexagon though. The way the hexagon is oriented in geom_hex the vertical parameter is vertex to vertex, whereas the horizontal parameter is side to side.

Here are three helper functions. First, wd_hex gives you a horizontal width length given the vertical parameter. So if you wanted your hexagon to be vertex to vertex to be 1000 meters (so a side is 500 meters), wd_hex(1000) returns just over 866. Second, if for your map you wanted to convert the numbers to densities per unit area, you can use hex_area to figure out the size of your hexagon. Going again with our 1000 meters vertex to vertex hexagon, we have a total of hex_area(1000/2) is just under 650,000 square meters (or about 0.65 square kilometers).

For maps though, I think it makes the most sense to set the hexagon to a particular area. So hex_dim does that. If you want to set your hexagons to a square kilometer, given our projected data is in meters, we would then just do hex_dim(1000^2), which with rounding gives us vert/horz measures of about (1241,1075) to supply to geom_hex.

#ggplot geom_hex you need to supply height and width
#if you want a regular hexagon though, these
#are not equal given the default way geom_hex draws them
#https://www.varsitytutors.com/high_school_math-help/how-to-find-the-area-of-a-hexagon

#get width given height
wd_hex <- function(height){
  tri_side <- height/2
  sma_side <- height/4
  width <- 2*sqrt(tri_side^2 - sma_side^2)
  return(width)
}

#now to figure out the area if you want
#side is simply height/2 in geom_hex
hex_area <- function(side){
  area <- 6 * (  (sqrt(3)*side^2)/4 )
  return(area)
}

#So if you want your hexagon to have a regular area need the inverse function
#Gives height and width if you want a specific area
hex_dim <- function(area){
  num <- 4*area
  den <- 6*sqrt(3)
  vert <- 2*sqrt(num/den)
  horz <- wd_hex(height)
  return(c(vert,horz))
}

my_dims <- hex_dim(1000^2)   #making it a square kilometer
sqrt(hex_area(my_dims[1]/2)) #check to make sure it is square km
#my_dims also checks out with https://hexagoncalculator.apphb.com/

Now onto the good stuff. I tend to think discrete bins make nicer looking maps than continuous fills. So through some trial/error you can figure out the best way to make those via cut. Also I make the outlines for the hexagons thin and white, and make the hexagons semi-transparent. So you can see the outline for the city. I like how by default areas with no shootings are not given any hexagon.

lev_cnt <- seq(0,225,25)
shoot_count <- base_map + 
               geom_hex(data=shoot, color='white', alpha=0.85, size=0.1, binwidth=my_dims, 
                        aes(x=x,y=y,fill=cut(..count..,lev_cnt))) + 
               scale_fill_brewer(name="Count Shootings", palette="OrRd")

We have come so far, now to automate exporting the figure to a PNG file. I’ve had trouble getting journals recently to not bungle vector figures that I forward them, so I am just like going with high res PNG to avoid that hassle. If you render the figure and use the GUI to export to PNG, it won’t be as high resolution, so you can often easily see aliasing pixels (e.g. the pixels in the North Arrow for the earlier base map image).

png('Philly_ShootCount.png', height=5, width=5, units="in", res=1000, type="cairo") 
shoot_count
dev.off()

Note the font size/location in the exported PNG are often not quite exactly as they are when rendered in the RGUI window or RStudio on my windows machine. So make sure to check the PNG file.

Weighted buffers in R

Had a request not so recently about implementing weighted buffer counts. The idea behind a weighted buffer is that instead of say counting the number of crimes that happen within 1,000 meters of a school, you want to give events that are closer to the school more weight.

There are two reasons you might want to do this for crime analysis:

  • You want to measure the amount of crime around a location, but you rather have a weighted crime count, where crimes closer to the location have a greater weight than those further away.
  • You want to measure attributes nearby a location (so things that predict crime), but give a higher weight to those closer to a location.

The second is actually more common in academic literature — see John Hipp’s Egohoods, or Liz Groff’s work on measuring nearby to bars, or Joel Caplan and using kernel density to estimate the effect of crime generators. Jerry Ratcliffe and colleagues work on the buffer intensity calculator is actually the motivation for the original request. So here are some quick code snippets in R to accomplish either. Here is the complete code and original data to replicate.

Here I use over 250,000 reported Part 1 crimes in DC from 08 through 2015, 173 school locations, and 21,506 street units (street segment midpoints and intersections) I constructed for various analyses in DC (all from open data sources) as examples.

Example 1: Crime Buffer Intensities Around Schools

First, lets define where our data is located and read in the CSV files (don’t judge me setting the directory, I do not use RStudio!)

MyDir <- 'C:\\Users\\axw161530\\Dropbox\\Documents\\BLOG\\buffer_stuff_R\\Code' #Change to location on your machine!
setwd(MyDir)

CrimeData <- read.csv('DC_Crime_08_15.csv')
SchoolLoc <- read.csv('DC_Schools.csv')

Now there are several ways to do this, but here is the way I think will be most useful in general for folks in the crime analysis realm. Basically the workflow is this:

  • For a given school, calculate the distance between all of the crime points and that school
  • Apply whatever function to that distance to get your weight
  • Sum up your weights

For the function to the distance there are a bunch of choices (see Jerry’s buffer intensity I linked to previously for some example discussion). I’ve written previously about using the bi-square kernel. So I will illustrate with that.

Here is an example for the first school record in the dataset.

#Example for crimes around school, weighted by Bisquare kernel
BiSq_Fun <- function(dist,b){
    ifelse(dist < b, ( 1 - (dist/b)^2 )^2, 0)
    }

S1 <- t(SchoolLoc[1,2:3])
Dis <- sqrt( (CrimeData$BLOCKXCOORD - S1[1])^2 + (CrimeData$BLOCKYCOORD - S1[2])^2 )
Wgh <- sum( BiSq_Fun(Dis,b=2000) )

Then repeat that for all of the locations that you want the buffer intensities, and stuff it in the original SchoolLoc data frame. (Takes less than 30 seconds on my machine.)

SchoolLoc$BufWeight <- -1 #Initialize field

#Takes about 30 seconds on my machine
for (i in 1:nrow(SchoolLoc)){
  S <- t(SchoolLoc[i,2:3])
  Dis <- sqrt( (CrimeData$BLOCKXCOORD - S[1])^2 + (CrimeData$BLOCKYCOORD - S[2])^2 )
  SchoolLoc[i,'BufWeight'] <- sum( BiSq_Fun(Dis,b=2000) )
}

In this example there are 173 schools and 276,621 crimes. It is too big to create all of the pairwise comparisons at once (which will generate nearly 50 million records), but the looping isn’t too cumbersome and slow to worry about building a KDTree.

One thing to note about this technique is that if the buffers are large (or you have locations nearby one another), one crime can contribute to weighted crimes for multiple places.

Example 2: Weighted School Counts for Street Units

To extend this idea to estimating attributes at places just essentially swaps out the crime locations with whatever you want to calculate, ala Liz Groff and her inverse distance weighted bars paper. I will show something alittle different though, in using the weights to create a weighted sum, which is related to John Hipp and Adam Boessen’s idea about Egohoods.

So here for every street unit I’ve created in DC, I want an estimate of the number of students nearby. I not only want to count the number of kids in attendance in schools nearby, but I also want to weight schools that are closer to the street unit by a higher amount.

So here I read in the street unit data. Also I do not have school attendance counts in this dataset, so I just simulate some numbers to illustrate.

StreetUnits <- read.csv('DC_StreetUnits.csv')
StreetUnits$SchoolWeight <- -1 #Initialize school weight field

#Adding in random school attendance
SchoolLoc$StudentNum <- round(runif(nrow(SchoolLoc),100,2000)) 

Now it is very similar to the previous example, you just do a weighted sum of the attribute, instead of just counting up the weights. Here for illustration purposes I use a different weighting function, inverse distance weighting with a distance cut-off. (I figured this would need a better data management strategy to be timely, but this loop works quite fast as well, again under a minute on my machine.)

#Will use inverse distance weighting with cut-off instead of bi-square
Inv_CutOff <- function(dist,cut){
    ifelse(dist < cut, 1/dist, 0)
}

for (i in 1:nrow(StreetUnits)){
    SU <- t(StreetUnits[i,2:3])
    Dis <- sqrt( (SchoolLoc$XMeters - SU[1])^2 + (SchoolLoc$YMeters - SU[2])^2 )
    Weights <- Inv_CutOff(Dis,cut=8000)
    StreetUnits[i,'SchoolWeight'] <- sum( Weights*SchoolLoc$StudentNum )
}   

The same idea could be used for other attributes, like sales volume for restaurants to get a measure of the business of the location (I think more recent work of John Hipp’s uses the number of employees).

Some attributes you may want to do the weighted mean instead of a weighted sum. For example, if you were using estimates of the proportion of residents in poverty, it makes more sense for this measure to be a spatially smoothed mean estimate than a sum. In this case it works exactly the same but you would replace sum( Weights*SchoolLoc$StudentNum ) with sum( Weights*SchoolLoc$StudentNum )/sum(Weights). (You could use the centroid of census block groups in place of the polygon data.)

Some Wrap-Up

Using these buffer weights really just swaps out one arbitrary decision for data analysis (the buffer distance) with another (the distance weighting function). Although the weighting function is more complicated, I think it is probably closer to reality for quite a few applications.

Many of these different types of spatial estimates are all related to another (kernel density estimation, geographically weighted regression, kriging). So there are many different ways that you could go about making similar estimates. Not letting the perfect be the enemy of the good, I think what I show here will work quite well for many crime analysis applications.