An effect size is a numerical measure of the magnitude of a difference, association, or other statistical effect. Unlike a P-value, it is intended to describe how large an effect is rather than only how strongly the data contradict a null hypothesis.
For two populations with means 
and 
and a common standard
deviation 
,
a standardized mean difference is
 |
Other effect sizes include a correlation coefficient, an odds ratio, and Cramér's V. The practical importance of a numerical effect size depends on the subject matter and cannot be inferred from its magnitude alone.
