The 'ministerial' model is the formal, legalistic premise that police are bound by the letter of the law, and the 'police justice' model defines police as highly trained experts capable of making complex decisions about how best to produce justice. The 'professional crime control' model views police as professional controllers of crime as a disease. Ignorance of the consequences of police discretion is irrelevant to the ministerial model and is a convenient state of knowledge for the police justice model. For the professional crime control model, however, ignorance of the consequences of police discretion is analogous to ignorance of the effects of a new drug. Knowing the average effects of a police action on all kinds of people is irrelevant to the ministerial model, but it would be helpful for the police justice model, although hardly a binding guide for action. Under the professional crime control model, such information would be an obligatory guide to practice. Knowing the differential effects of the same police action on different kinds of people in various circumstances would again be irrelevant for the ministerial model but would aid the police justice model in determining the deterrent effect of various police decisions. The professional crime control model would find such information similarly useful but would require a higher standard of proof than the police justice model. 55 footnotes.