In a 1997 paper on the effect of police hiring on crime rates, Levitt used the timing of mayoral and gubernatorial elections as an instrumental variable to identify a causal effect of police on crime. Past studies had been inconclusive because of the simultaneity inherent in police hiring (when crime increases, more police are hired to combat crime). The findings of this paper were found to be the result of a programming error. This was pointed out in a comment by Justin McCrary published in the "American Economic Review" in 2002. In a response published with McCrary's comment Levitt admits to the error and then goes on to offer alternative evidence to support his original conclusions. Levitt's 1997 paper was also criticized in another comment that demonstrates the weakness of the instrumental variables used in the original study, rendering the interpretation difficult if not impossible.