This paper introduces education researchers to a new way of measuring the effects of "whole-school" reforms on student achievement. Based on interrupted time-series analysis, the method estimates a reform's effects by comparing standardized test scores in a given grade before and after the reform is launched. Data from MDRC's evaluation of the Accelerated Schools model serve to illustrate how the method works. To help education researchers apply the approach in their own research, the paper offers a conceptual rationale for using it, reviews its strengths and limitations, and describes the statistical procedures it entails.