Prediction of Crime and Recidivism

There are four statistical outcomes that can occur when one is faced with making a prediction of any kind of future behavior. One can either predict that the behavior, in this case crime, will take place or that it will not take place. At the end of some specified period, one observes whether the predicted behavior actually has taken place or has not taken place.

If one predicts that crime will take place and later finds that this has indeed happened, the prediction is called a true positive. One has made a positive prediction and it has turned out to be correct, or true. Similarly, if one predicts that crime will not take place and it in fact does not, the prediction is called a true negative, since one has made a negative prediction of crime and it turned out to be true. These, of course, are the two outcomes that one wishes to maximize in making predictions.

There are also two kinds of mistakes that can be made. If one predicts that crime will take place and it does not, the outcome is called a false positive. A positive prediction was made and it turned out to be incorrect, or false. In practice, this kind of mistake usually means that a person has been unnecessarily detained to prevent a crime that would not have taken place in any event. If one predicts that violence will not take place and it does, the outcome is called a false negative. In practice, this kind of mistake often means that someone who is not detained, or who is released from detention, commits a criminal act in the community. Obviously, predictors of violence try to minimize these two outcomes.

Decision rules. Decision rules involve choosing a "cutting score" on some predictive scale, above which one predicts, for the purpose of intervention, that an event will happen. A cutting score is simply a particular point on some objective or subjective scale. When one sets a thermostat at 68°, for example, one is establishing a cutting score for the operation of a heating unit. When the temperature drops below 68° the heat comes on, and when it goes above 68° the heat goes off. The "beyond a reasonable doubt" standard of proof in the criminal law is a cutting score for the degree of certainty that a juror must have in order to vote for conviction. Conviction is to take place only if doubt is "unreasonable." In the context of parole prediction, one could state that if a prisoner has a higher than X probability of recidivism, he or she should be denied parole for a given period.