The purpose of police work and prosecution is crime control.Moreover, the figures the authorities cite may actually reflect failure, not success. More arrests and convictions could indicate more crime, not less. To evaluate the effectiveness of any crime control measure, you need to look at the bottom line, i.e., the number/severity of crimes committed, or at least some good proxy thereof. More simply put: you need to measure outputs, not inputs.
But what police and prosecutors do from day to day is make arrests and secure convictions (or guilty pleas) and thus sentences. It seems natural to count those activities and use the counts as performance measures. That, however, turns out to be a mistake. Actual arrests and prosecutions are mostly costs rather than benefits.
Here’s my favorite example of a crime-control measure that is evaluated using exactly the wrong data: gun buy-back programs. Politicians regularly tout the number of guns collected as evidence of the programs’ success:
Congressman Patrick J. Kennedy: “It is obvious that with 1,235 guns off the streets, the program is working effectively.” (1,235 was the number of guns turned in during a buy-back.)The problem, of course, is that purchasing a gun doesn’t necessarily mean reducing the number of guns in the hands of the public. The gun buy-back might encourage the trafficking of guns into the region, if the price is high enough. If the price is low, it still might encourage people to trade-in old guns so they can purchase new ones (much like when an auto dealership offers to buy your old junker). And even if gun buy-backs succeed in reducing the stock of guns in the hands of the public, who’s selling them – criminals, whose livelihoods depend on their guns, or law-abiding citizens who rarely need their guns? While it’s possible that gun buy-backs reduce crime, the number of guns collected does nothing to prove it.
ABQ Journal: “There are fewer guns in Las Cruces today. Police collected about 160 weapons and 1,000 rounds of ammunition during a gun buy-back program Saturday.”
Website of Maricopa County (AZ) Supervisor Mary Rose Garrido Wilcox: “Gun Buy-Back a huge success … ‘I'm excited to announce that this year we took in more than 200 guns at our two locations. …,’ said Wilcox. … ‘The Gun Buy-Back program has saved hundreds of lives in these communities and in our own. We are dedicated to continue saving lives of Arizonans by getting guns off the street,’ said Supervisor Wilcox.”
District of Columbia Police Department Press Release: “Saying that lives will be saved as a result, Chief of Police Charles H. Ramsey today announced the Metropolitan Police Department took in 1,787 firearms on Thursday, Friday and Saturday during the first phase of ‘Operation Save A Life,’ the District of Columbia's gun buy-back program. … ‘Every one of the nearly 1,800 guns collected represents a step towards making the District of Columbia safer, and this weekend's buy-back served both as a national model and as an inspiration for buy-backs HUD is supporting in other communities throughout the nation,’ he said.”
Another press release from the same source: “‘Getting one dangerous and illegal firearm off our streets or out of a home is significant because of the potential pain and tragedy that single weapon can inflict,’ said Chief of Police Charles H. Ramsey. ‘Getting more than 6,200 firearms off our streets and out of our homes is a momentous victory for safer children, safer families and safer communities throughout our city.’”
When I think about gun buy-backs’ effectiveness, I recall the old story (possibly apocryphal, but still instructive) about the town of Abruzzi, Italy:
The city was plagued by vipers, and the city fathers determined to solve the problem by offering a reward for any viper killed. Alas, the supply of vipers increased. Townspeople had started breeding them in their basements. [from S. E. Rhoads (1985), The Economist's View of the World: Government, Markets, and Public Policy, Cambridge University Press, p. 58]And I’ll bet that one of the city fathers trumpeted the success of the program based on the number of dead vipers turned in for the bounty.