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Effect Size

  1. Robert J. Grissom

Published Online: 30 JAN 2010

DOI: 10.1002/9780470479216.corpsy0295

Corsini Encyclopedia of Psychology

Corsini Encyclopedia of Psychology

How to Cite

Grissom, R. J. 2010. Effect Size. Corsini Encyclopedia of Psychology. 1.

Author Information

  1. San Francisco State University

Publication History

  1. Published Online: 30 JAN 2010


Most broadly defined, an effect size measures the strength of association between variables or the degree of difference between parameters. If a null hypothesis is false, an effect size measures degree of falseness of that null hypothesis. In the planning of research, the anticipated effect size, or the effect size of minimum interest that the researcher wants to detect, is used in power analysis to estimate needed sample sizes. Estimates of effect size from related studies can be averaged in a meta-analysis in order to yield a better estimate than could be expected from any single study.


  • effect size;
  • strength of association;
  • proportion of variance explained