Meta-analysis:

research design that involves collecting a number of studies on a particular subject, then statistically averaging together the results of those studies to come up with an overall average measure. This can be used to make deductions about the effect under study. For example, if 20 randomized controlled trials were conducted on a new antidepressant drug, a meta-analysis could be used to determine if overall (i.e. averaged across all the trials) the drug had a positive effect.