Effect Sizes

#1
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.
 

hlsmith

Omega Contributor
#2
Classifications are arbitrary. A 0.1 may be super exciting to one person in their context and not in a different context. Think of saving lives, a little bit is still important, though if you decrease length of hospital stay by 15 minutes that may not mean much.
 

CowboyBear

Super Moderator
#3
Can someone explain (with an example) why a researcher would choose a "medium" effect size for example?
I'm not quite sure what you mean by "choose". Do you mean "Why would a researcher interpret an effect size of (say) d = 0.5 as being "medium" in size? Or do you mean, why would a researcher hypothesise a "medium" effect for power analysis? Or do you mean something else?
 
#4
I'm not quite sure what you mean by "choose". Do you mean "Why would a researcher interpret an effect size of (say) d = 0.5 as being "medium" in size? Or do you mean, why would a researcher hypothesise a "medium" effect for power analysis? Or do you mean something else?
Good point. Bad wording on that. What I've been reading is researchers hypothesizing effect sizes for studies, but they offer no reason for that hypothesis. It has to be based on something... I know each study will vary, obviously, but how are they deciding that?
 

CowboyBear

Super Moderator
#5
Welll... Cohen's original effect sizes were based on his rough experience of what was most common in psychology. So hypothesising a "medium" effect size is like hypothesising a typical, expected effect size (although in reality Cohen's medium effect size is a bit higher than the average in psychology - see Richard et al.). But most likely the researcher just can't be bothered coming up with a more specific hypothesis, and is going with convention.