Statistical notes for clinical researchers: effect size

  • Kim, Hae-Young (Department of Health Policy and Management, College of Health Science, and Department of Public Health Sciences, Graduate School, Korea University)
  • Published : 2015.11.30


Though p values give information on statistical significance, they are confounded with the sample size. Effect size can make up the weak point, by providing information on the actual effect which is independent of the sample size. Therefore, reporting the effect size as well as the p value is recommended.



  1. Coe R: It's the effect size, stupid: what effect size is and why it is important. Paper presented at the 2002 Annual Conference of British Education Research Association, University of Exeter, Exeter, Devon, England, September 12-14, 2002. Available from: (updated 2015 Sep 6).
  2. Sullivan GM, Feinn R. Using effect size - or why p value is not enough. J Grad Med Educ 2012;4:279-282.
  3. Becker LA: Effect size (ES). Available from: (updated 2015 Sep 6).

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