The goal of this paper is to introduce education researchers to a new approach for measuring the impact of whole-school reform. The approach is based on interrupted time-series analysis, which has been used to evaluate programs in many fields, but has not been used widely to study education initiatives. The application presented measures program impacts on student performance by comparing standardized test scores for a number of annual student cohorts in a specific grade after a reform is launched (its followup period) with the scores of cohorts from several years before the reform was launched (its baseline period). The approach is used to measure impacts on three facets of student performance: (1) average (mean) test scores, which summarizes impacts on total performance; (2) the distribution of scores across specific ranges, which helps identify where in the distribution of student performance impacts were experiences; and (3) the variation (standard deviation) of scores, which indicates how the disparity in student performance was affected. To help researchers use the approach, the paper lays out its conceptual rationale, describes its statistical procedures, explains how to interpret its findings, indicates its strengths and limitations, and illustrates how it was used to evaluate a major whole-school reform modelAccelerated Schools. (Contains 4 tables, 3 figures, and 24 references.) (Author/SLD)