Effect sizes are measures of the strength of effects. In terms of the training course example, the effect size measures how much the training program improved test scores.
Cohen’s d is a measure of effect size that is discussed in the tutorial.
The basic formula for d is:
This corresponds to the differences between means divided by the standard deviation.
Cohen offered the following rough guidelines to help you interpret effect sizes: a small ES = .2, a medium ES = .5, and a large ES = .8 and above.