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A measure of effect size for experimental designs with heterogeneous variances
Janis F Johnston1, Kenneth J Berry, Paul W Mielke
1Department of Sociology, Colorado State University, Fort Collins, CO 80523-1784, USA. janisj@lamar.colostate.edu
Abstract:
A recent trend in the psychological literature has been to include measures of effect size when reporting probability values. The several measures of effect size associated with the Student t test for two independent samples are appropriate only when the variances are homogeneous. In this paper, commonly used measures of effect size are considered and compared, using four data sets. A chance-corrected measure of effect size is provided for two or more treatment groups characterized by either homogeneous or heterogeneous variances.
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