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A meta-analysis is a statistical practice of combining the results of a number of studies. The first meta-analysis was performed by Karl Pearson in 1904, in an attempt to overcome the problem of reduced statistical power in studies with small sample sizes; analyzing the results from a group of studies can allow more accurate estimation of effects. Meta-analysis is a collection of systematic techniques for resolving apparent contradictions in research findings. Meta-analysts translate results from different studies to a common metric and statistically explore relations between study characteristics and findings.
Although meta-analysis is widely used in evidence-based 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.
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2 Outspoken critics
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