Testing for Structural Breaks in the Evaluation of Programs.

A youth homicide reduction initiative in Boston in the mid-1990s poses particular difficulties for program evaluation because it did not have a control group and the exact implementation date is unknown. A standard methodology in program evaluation is to use time series variation to compare pre- and post-program outcomes. Such an approach is not valid, however, when the timing of a potential break is unknown. To evaluate the Boston initiative, we adapt from the macroeconomics literature a test of unknown break point to test for a change in regime. Tests for parameter instability provide a flexible framework for testing a range of hypotheses commonly posed in program evaluation. These tests both pinpoint the timing of maximal break and provide a valid test of statistical significance. We evaluate the results of the estimation using the asymptotic results in the literature and with our own Monte Carlo analyses. We conclude there was a statistically significant discontinuity in youth homicide incidents (on the order of 60 percent) shortly after the intervention was unveiled.