ProPublica’s Misleading Machine Bias

A May 2016 report by ProPublica revealed a stark finding. A commonly used statistical tool used by criminal justice professionals to predict criminal recidivism and determine sentencing guidelines was inherently biased.

The program, Compas, supposedly slanted its predictions to assume African-American were more likely to commit another crime than Caucasians. Not just slightly, but substantially so. Their analysis accuses the software of labeling black defendants as future criminals at twice the rate as whites, and whites were more often labeled as low risk than black defendants.

But the numbers for those statistics appear to not be based on output of Compas’ model. 

In the notebook for ProPublica’s calculations (cells 50–55), the percentages are calculated using a number of values for whether the perpetrator recommitted a crime, how much time they spent in jail, and whether their score was valid—but not the actual value of their Compas score.

The numbers presented for how the model predicts differently for whites versus blacks is based on a calculation that doesn’t involve Compas’ model.

An analysis of the Compas data by Investigative Economics shows relatively consistent accuracy for each race defined, between 59% and 67%.

While there are some inconsistencies across race and ethnicity, predictions for African-Americans are both accurate and precise, more so than for other groups. Asians and Native Americans are an exception as there are few data points to work with.

Compas Model and Predictive Parity

A common measurement of fairness in analytics is predictive parity — whether the total number predicted for a sensitive attribute like race aligns with actual totals.

That is, the percentage of African Americans predicted to recommit crimes is similar to the actual percentage of African Americans who commit crimes.

Even if the model is inaccurate for certain populations, predictive parity shows that it’s not erroneously predicting certain populations to be criminals far beyond what they are in reality, which would be a sign that the model is unfairly skewed against that class.

And the predictive parity for African Americans is close — within 2% of actual rates. More so than for other groups. Which is exactly what Northpointe, the maker of Compas software, claimed in their defense of the program.

One point about the ProPublica criticism did appear correct. Caucasians and Hispanics are predicted to recommit crimes at a substantially lower rate than they actually do.

For Caucasians, they are predicted to recommit 25% of the time, but they actually recommit 39% of the time. And the accuracy of predictions for both Caucasians and Hispanics are substantially lower than for African Americans.

But the higher accuracy and precision in the African American population undercuts the idea that they are being unfairly biased.