Effect size

Effect size is a way of quantifying the effectiveness of a particular intervention, relative to some comparison. It is the standardized mean difference between the two groups E (experimental) and C (control):

In studies where there is a large control group, its SD should be used in calculation of effect size. If there is not a true control group, or the control group is small, it is better to use a pooled estimate of SD:

, where NE and NC are the sizes of experimental and control groups, respectively.


Coe R. It’s the Effect Size, Stupid. What effect size is and why it is important. Paper presented at the Annual Conference of the British Educational Research Association, University of Exeter, England, 12-14 September 2002.

Cumming G. Understanding The New Statistics - Effect Sizes, Confidence Intervals, and Meta-Analysis. Routledge, Taylor & Francis, 2012.

Ellis PD. The essential Guide to Effect Sizes - Statistical Power, Meta-Analysis, and the Interpretation of Research Results. Cambridge University Press, 2010.

Nakagawa S, Cuthill IC. Effect size, confidence interval and statistical significance: a practical guide for biologists. Biol Rev. 2007; 82: 591-605.

Sullivan GM, Feinn R. Using effect size - or why the p value is not enough. J Grad Med Educ. 2012; 4(3): 279-282.

Tags: , ,

Created at: 2014-02-06
Updated at: 2015-07-27
Written by: Vesa Oikonen