You might want to take a look at the link I posted in this thread, which compares and contrasts “pooling” ie. treating all of the reports as a single large trial, and “combining” – ie. averaging in proportion to sample size.
Pooling studies by treating the data of the individual articles as if it came from one large trial, is not valid in the general case due to Simpson’s paradox. The paper in the link is open access and goes into the mathematical details.
You might want to check out my thread on meta-analysis, because there are a lot of techniques recommended in the secondary sources that do not stand up to mathematical scrutiny.
I really should organize these better, but there are a lot of useful papers here.