Statistical power for nonequivalent pretest-posttest designs - The impact of change-score versus ANCOVA models

Nonequivalent controlled pretest-posttest designs are central to evaluation science, yet no practical and unified approach for estimating power in the two most widely used analytic approaches to these designs exists. This art icle fills the gap by presenting and comparing useful, unified power formul as for ANCOVA and change-score analyses, indicating the implications of eac h on sample-size requirements. The authors close with practical recommendat ions for evaluators. Mathematical details and a simple spreadsheet approach are included in appendices.