The base rates in the diagram are not historical but "potential" rates. They show the proportion of current inmates up for parole who would be re-arrested if paroled. In practice this is indeed estimated by looking at historical rates but as long as the true base rates are different in reality, no algorithm can be fair in the two senses described above.

The base rates in the diagram are not historical but "potential" rates. They show the proportion of current inmates up for parole who would be re-arrested if paroled. In practice this is indeed estimated by looking at historical rates but as long as the true base rates are different in reality, no algorithm can be fair in the two senses described above.