Subgroups Cochran's Q test I'm actually doing a meta-analysis to investigate the effect of an instructional method on learners' performance. I have calculated coehen's d and Hedges' g, the average global size, W (weight) for each study and cochran's Q test using : Q=wi(teta(i) - teta (average))2. I performed a grouping (moderators) and I have to calculate Q for each group. Anyone knows PLZ how to do it ? which formulate to use ?? 
P.S: I'm a computer science phd student, it's the first time I deal with such studies and I'm really lost.
 A: The Q test has low power to reject the hypothesis that the studies are homogenous and this is particularly true if there are a small number of studies in the meta-analysis. Although people often break up their dataset into smaller and smaller sub-groups this is not a good idea as the estimates become more unstable. It is better to fit an overall model to all the studies and then introduce the moderator(s) in a meta-regression. This then gives an estimate for each level of a categorical moderator and by comparing with the model without moderator you can tell how much the moderator has reduced heterogeneity.
I use R and I would use the metafor package for this but you can use Stata
You have the choice between fitting a fixed effect model and a random effect model. The fixed effect model assumes there is a single underlying true effect and it is your job to estimate it. The random effect model assumes that there is no true singel effect but instead a distribution of them and it is your job to estimate the mean and variance of that distribution. You will often see statements that the choice should be based on the amount of heterogeneity observed but this is not strictly correct. Adding a moderator does not really change the principles. In your case you know the science better than I do but I would have thought that a random effect model was more appropriate here.
You might benefit from (re-)reading some introductory material on meta-analysis. The tag meta-analysis has some references, many of them can be freely downloaded.
