Professor of Sociology

Effect Sizes

I regularly forget standard rules of interpretation for effect sizes when running analyses. I figured I’d put these here so I can quickly refer to them when needed. For more details on most effect sizes, Wikipedia is always useful.

Cohen’s d

Cohen’s d is typically used when doing t-tests. The basic calculation is the difference between two means divided by a standard deviation (usually the pooled standard deviation). Here’s the interpretation.

Effect Sized
Very small0.01
Small0.20
Medium0.50
Large0.80
Very Large1.20
Huge2.00

The standard citations are to Jacob Cohen’s work:
Cohen, Jacob. 1988. Statistical Power Analysis for the Behavioral Sciences. Mahwah, N.J.: Lawrence Erlbaum.
Cohen, Jacob. 1992. “A Power Primer.” Psychological Bulletin 112(1):155–59.

However, the new citation is to Sawilowsky:
Sawilowsky, Shlomo S. 2009. “New Effect Size Rules of Thumb.” Journal of Modern Applied Statistical Methods 8(2):597–99.

Pearson’s r

Pearson’s r is a standard measure of correlation. It can also be calculated for other statistical tests.

Effect Sizer
Small0.10
Medium0.30
Large0.50

These interpretations also come from Jacob Cohen’s work:
Cohen, Jacob. 1988. Statistical Power Analysis for the Behavioral Sciences. Mahwah, N.J.: Lawrence Erlbaum.