Determining the power of multiple regression analyses both with and without repeated measures.

Power analysis guides researchers in planning how much data to collect. This article describes BW-Power, a computer program for the Windows 95 environment that performs power analyses for research designs that may or may not include both between- and within-subjects factors. We discuss how BWPower easily accommodates both between- and within-subjects factors and provide examples of BWPower's use in performing power analyses on designs with only between-subjects factors, designs with only repeated measures, and with mixed between- and within-subjects designs. We highlight the major features of BWPower's user interface, such as the ability to iteratively increment or decrement the number of subjects and the automatic recalculation of power when the number of subjects or effect sizes is changed.