Introduction to research: Effect sizes

What’s the idea? An effect size is a “way of quantifying the difference between two groups” (Coe, 2017, p. 339). It allows us to move beyond the question of whether or not an intervention works and toward an understanding of how well it works in different contexts. What does it mean? The formula for an effect size is as follows: Effect size = (mean of experimental group) – (mean of the control group) divided by standard deviation (SD) The following is an effect size calculation, showing how time of day affects children’s learning (Coe, 2002, referring to Dowson, 2002): Half of the children in a class were randomly allocated to listen to a story and respond to questions at 9am The other half of the children listened to the same story and responded to questions at 3pm The children’s understanding of the story was assessed by using a test where the number of correct answers was measured out of 20 The morning group had an average of score of 15.2, whereas t