Instead of having so much code just randomly floating around in blog posts, I need to start making packages (both in R and python) more often. I took it as a challenge to make a simple python package, here *retenmod* (pypi, github). I got the idea after answering a question on crossvalidated. (The resources I leveraged the most were these two sites/tutorials, packaging projects and minimal example.)

It is a simple port of the R package `foretell`

that provides several different models to forecast churn based on aggregate survival probabilities. So it only has three functions, and I did not focus too much on extras (like building sphinx docs). Buit it has just the amount of complexity to make a nice intro get my feet wet example.

So you can now download/install the package via pip:

`pip install retenmod`

And it will automatically install scipy and numpy if you do not have them already installed. For a very simple example, I don’t have retention probabilities for any police department offhand, but this document has estimates for how many staff positions police tend to retain after increases.

Here is a simple example of using the library, in particular the BdW model.

```
import retenmod
import matplotlib.pyplot as plt
large = [100,66,56,52,49,47,44,42]
time = [0,1,2,3,4,5,10,15]
# Only fitting with the first 3 values
train, ext = 4, 15
lrg_bdw = retenmod.bdw(large[0:train],ext - train + 1)
# Showing predicted vs observed
pt = list(range(16))
fig, ax = plt.subplots()
ax.plot(pt[1:], lrg_bdw.proj[1:],label='Predicted',
c='k', linewidth=2, zorder=-1)
ax.scatter(time[1:],large[1:],label='Observed',
edgecolor='k', c='r', s=50, zorder=1)
ax.axvline(train - 0.5, label='Train', color='grey',
linestyle='dashed', linewidth=2, zorder=-2)
ax.set_ylabel('% Retaining Position')
ax.legend(facecolor='white', framealpha=1)
plt.xticks(pt[1:])
plt.show()
```

So you can see even with only fitting the data to the first three years, years 4 and 5 were forecasted quite well. It underestimates retention further out at 10 and 15 years (the model has a hard time going down very fast from 100 to 66 and then flattening out in a reasonable way). But even so the super far out forecasts are not that crazy given only three data points.

I will have to work on an example later of showing how to translate this to cost-benefit analysis (although would prefer actual retention data from a PD). Essentially you can calculate the benefit of trying to save officers (retain them) vs hiring new officers and training them up based on just aggregate data. If you wanted to do something like estimate if retention is going down due to recent events, I would probably use micro-level data and estimate a survival model directly.

Next up I will try to turn my Exact distribution tests (R Code) for day of week/Benford’s analysis into a simple R package and see if I can get it on Cran. Posting to pypi is quite easy.

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