Question

Explain what a meta-analysis is (i.e., what is a meta-analysis in terms of effect sizes).

Answer 1

Meta" is Greek prefix meaning "after" or "beyond." Meta-analysis (MA) is a secondary analysis after other researchers had done their own analyses and the meta-analyzer can go beyond what had been accomplished in the past. Simply put, MA is analysis of analyses. Even if you might never done MA before, you have already done quasi-meta-analysis, namely, literature review, which is typically presented in the following way:

• According to So and So (2009), Program X can effectively reduce the risk of colon cancer.
• Smith and Smith (2010) found that patients were unresponsive to Treatment X. It is a waste of your money.
• Prior research found that the therapeutic effect of Program X is minimal (Doe and Doe, 2014).

What can be done with the preceding heterogeneous or even contradictory findings? The answer is: One must go beyond literature review by looking into MA and effect size.

Effect size can be conceptualized as a standardized difference. In the simplest form, effect size, which is denoted by the symbol "d", is the mean difference between groups in standard score form i.e. the ratio of the difference between the means to the standard deviation. This concept is derived from a school of methodology named Meta-analysis, which was developed by Glass (1976).