This may seem like a ridiculous question, but it's bugging me and no text book or paper I've read has answered it. I am looking into research on effect sizes and their use in research. I see a lot of mention of Cohen's D and F2 and the "small", "medium" and "large" effect sizes. Can someone explain (with an example) why a researcher would choose a "medium" effect size for example? I'm just not seeing any justifications and I know it isn't just an arbitrary decision.