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An alternative to Cohen's standardized mean difference effect size: a robust parameter and confidence interval in the
James Algina1, H J Keselman, Randall D Penfield
1Department of Educational Psychology, University of Florida, Gainesville, FL 32611-7047, USA. algina@ufl.edu
Psychological Methods
|October 14, 2005
Summary
A new, robust effect size measure using trimmed and Winsorized means offers better population separation than Cohen
Area of Science:
- Statistics
- Psychometrics
- Data Analysis
Background:
- Cohen's effect size is a standard measure of population separation.
- Limitations exist in Cohen's effect size when dealing with non-normal distributions.
Purpose of the Study:
- To introduce and evaluate a robust effect size measure.
- To compare the performance of confidence intervals for the new effect size measure.
Main Methods:
- Developed a robust effect size using 20% trimmed means and Winsorized variance.
- Investigated confidence interval coverage probabilities using noncentral t-distribution and percentile bootstrap methods.
Main Results:
- The robust effect size measure demonstrated improved population separation.
- Percentile bootstrap confidence intervals exhibited better coverage probability across various distributions and effect sizes compared to noncentral t-distribution intervals.
Conclusions:
- The proposed robust effect size is a superior alternative to Cohen's effect size for assessing population separation.
- Percentile bootstrap confidence intervals are recommended for their improved accuracy in statistical inference.
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