ProPublica's COMPAS Data Revisited

Matias Barenstein

I examine the COMPAS recidivism risk score and criminal history data collected by ProPublica in 2016 that fueled intense debate and research in the nascent field of 'algorithmic fairness'. ProPublica's COMPAS data is used in an increasing number of studies to test various definitions of algorithmic fairness. This paper takes a closer look at the actual datasets put together by ProPublica. In particular, the sub-datasets built to study the likelihood of recidivism within two years of a defendant's original COMPAS survey screening date. I take a new yet simple approach to visualize these data, by analyzing the distribution of defendants across COMPAS screening dates. I find that ProPublica made an important data processing error when it created these datasets, failing to implement a two-year sample cutoff rule for recidivists in such datasets (whereas it implemented a two-year sample cutoff rule for non-recidivists). When I implement a simple two-year COMPAS screen date cutoff rule for recidivists, I estimate that in the two-year general recidivism dataset ProPublica kept over 40% more recidivists than it should have. This fundamental problem in dataset construction affects some statistics more than others. It obviously has a substantial impact on the recidivism rate; artificially inflating it. For the two-year general recidivism dataset created by ProPublica, the two-year recidivism rate is 45.1%, whereas, with the simple COMPAS screen date cutoff correction I implement, it is 36.2%. Thus, the two-year recidivism rate in ProPublica's dataset is inflated by over 24%. This also affects the positive and negative predictive values. On the other hand, this data processing error has little impact on some of the other key statistical measures, which are less susceptible to changes in the relative share of recidivists, such as the false positive and false negative rates, and the overall accuracy.

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