PSY 350 Lecture 29: PSY 350 3-3-18
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Common effect size index for comparing two groups d= (mean difference between groups)/(standard deviation) ^^^this takes the difference between groups and puts it into a well-known scale, so we could compare across studies. Cohen"s d tells us how different the unknown population is from the known population. If you are comparing a sample mean to a population mean: d= (m_(treatment )- _( no treatment))/ . If you are comparing two samples: d= (m_(treatment )- m_( no treatment))/ . *notice that the sample size does not appear anywhere in this formula d is independent of the sample size. But d alone can"t tell us whether our value is statistically significant. Therefore, we need to report both the significance test and the effect size in our results. Interpreting cohen"s d d = 0. 2 = a small effect d = 0. 5 = a medium effect d = 0. 8 = a large effect. *however, the best guide to this is context.
