Effect size is a statistic that describes the extent of the difference between the average score in one condition (often a control condition) compared with the average score in the other condition, for example performance after taking part in some sort of intervention. Effect size are calculated by taking the mean score for the control group away from the mean score for the experimental group and dividing by the standard deviation of all the scores combined. This means that an effect size of 0.8 would mean that the average person in the experimental group scores 0.8 of a standard deviation higher than the average person in the control group, or more than 79% of the sample, (Coe et al. 2002).

References

Coe, R. (2002). 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. Retrieved November 30, 2020, from http://www.leeds.ac.uk/educol/documents/00002182.htm