Work out effect size for study instantly with clear inputs, formula shown and shareable results.
Cohen's d standardises a mean difference by the pooled standard deviation. Means of 24.6 and 21.2 with standard deviations of 5.4 and 5.9 give a pooled SD of 5.66 and d = 0.60 - a medium effect, with the two distributions still overlapping about 76%. Cohen's 0.2, 0.5 and 0.8 benchmarks are conventions, not laws.
Cohen's d
d = (mean1 - mean2) / pooled standard deviation
Pooled standard deviation
sqrt((sd1^2 + sd2^2) / 2) for equal group sizes
Because a p value only says whether an effect exists, not how large it is. With a big sample a trivial difference can be highly significant.
No. Cohen himself called them arbitrary. In some fields d = 0.2 is a major finding; in others 0.8 is unremarkable.