In this meta analysis the authors quote an overall Hedge's g effect size of $g=1.116$ for the effect of transcranial magnetic stimulation (TMS) on cocaine and methamphetamine use. Seems dubious to me but I will ignore that for now.
I have been asked by a colleague to calculate the required sample size for a proposed study. The design is quite simple: testing difference in change in number of days per week of drug use from pre- to post-treatment between a control group and a TMS treatment group. I propose to use a simple non-paired t-test on change scores to do this.
I know how to calculate the required sample size for this analysis based on Cohen's d, but am unfamiliar with Hedge's g. From what I read here there are ways to calculate Cohen's d from Hedge's g, but there are a few things I don't understand
(a) I am unclear whether the Hedge's g used in meta-analyses to generalise effect sizes across multiple studies is the same as the effect size used in single studies as an alternative to Cohen's d for small sample sizes
(b) I do not know how to convert the Hedge's g effect size of $g=1.116$ to Cohen's d since the N they refer to in the paper seems to refer to studies rather than subjects
Any advice much appreciated.
