Prediction of Crime and Recidivism

The term base rate refers to the proportion of individuals in the group being examined who can be expected to engage in violent criminality. It is the average, or "chance," rate that prediction seeks to improve upon. Prediction schemes can be evaluated either in terms of how well they differentiate true and false positives or in terms of how much they improve on the base rates. Thus, in the Michigan parole prediction study discussed below, the base rate for violent recidivism among all persons released from prison was 10 percent. A prediction scale was devised that could identify one subgroup of which 40 percent committed a violent crime after release. This device, therefore, improved on the base rate by a factor of four, although 60 percent of the individuals predicted to be violent were still false positives.