A meta-analysis uses statistical methods to synthesize the results of multiple studies, often by calculating a weighted average of effect sizes. Before embarking on a meta-analysis, make sure you are familiar with reviews, in particular systematic reviews.
A forest plot is a type of graph used in meta-analyses that displays the results of multiple studies next to each other.
Practical Meta-Analysis by 
Meta-Analysis in Medicine and Health Policy by 
An Introduction to Systematic Reviews by 
Handbook of Meta-Analysis in Ecology and Evolution by 
