Although meta-analysis is widely used in medicine today, a meta-analysis of a medical treatment was not published till 1955. In the 1970s more sophisticated analytical techniques were introduced in educational research, starting with the work of Eugene V. Glass.
Because the results from different studies investigating different dependent variables are measured on different scales, the dependent variable in a meta-analysis is some standard measure of effect size, such as a standard score equivalent to a difference between means (d), or an odds ratio.
A weakness of the method is that sources of bias are not controlled by the method. A good meta-analysis of badly designed studies will still result in bad statistics.