Randomization Inference for Before-and-After Studies with Multiple Units: An Application to a Criminal Procedure Reform in Uruguay

We study the immediate impact of a new code of criminal procedure on crime. In November 2017, Uruguay switched from an inquisitorial system (where a single judge leads the investigation and decides the appropriate punishment for a particular crime) to an adversarial system (where the investigation is now led by prosecutors and the judge plays an overseeing role). To analyze the short-term effects of this reform, we develop a randomization-based approach for before-and-after studies with multiple units. Our framework avoids parametric time series assumptions and eliminates extrapolation by basing statistical inferences on finite-sample methods that rely only on the time periods closest to the time of the policy intervention. A key identification assumption underlying our method is that there would have been no time trends in the absence of the intervention, which is most plausible in a small window around the time of the reform. We also discuss several falsification methods to assess the plausibility of this assumption. Using our proposed inferential approach, we find statistically significant short-term causal effects of the crime reform. Our unbiased estimate shows an average increase of approximately 25 police reports per day in the week following the implementation of the new adversarial system in Montevideo, representing an 8 percent increase compared to the previous week under the old system.