Mark Ramseyer (Harvard), Eric Rasmusen (Indiana), and Manu Raghav (Wash. & Lee) recently circulated an interesting paper, Convictions versus Conviction Rates: The Prosecutor's Choice, that compares the two main ways many assess a prosecutor's effectiveness. Prosecutors' dual roles--deciding which cases to prosecute and convincing courts to convict criminal defendants--complicate assessment efforts. For many "it is natural to suppose that a prosecutor's conviction rate--the ratio of convictions to cases prosecuted--is a sign of his [or her] competence. Prosecutors, however, choose which cases to prosecute. If they prosecute only the strongest cases, they will have high conviction rates. Any system which pays attention to conviction rates, as opposed to the number of convictions, is liable to abuse." The paper explores the dilemma flowing from selection effect. From the abstract:
"We model the tradeoffs theoretically in two models, one of a benevolent social planner and one of a prosecutor rewarded directly for his conviction rate as well as caring about convictions and personal goals. We also look at anecdotal evidence from Japan and detailed U.S. data drawn from county-level crime statistics and a survey of all state prosecutors by district. We find that prosecution rates vary little with budget, but conviction rates do increase, and that the conviction rate declines in the number of cases prosecuted and with the crime rate of a district."