Simple pooling versus combining in meta-analysis.

The simple pooling of data is often used to provide an overall summary of subgroup data or data from a number of related studies. In simple pooling, data are combined without being weighted. Therefore, the analysis is performed as if the data were derived from a single sample. This kind of analysis ignores characteristics of the subgroups or individual studies being pooled and can yield spurious or counterintuitive results. In meta-analysis, data from subgroups or individual studies are weighted first, then combined, thereby avoiding some of the problems of simple pooling. The purpose of this article is to describe how simple pooling differs from meta-analysis, provide a detailed analysis of why simple pooling can be a poor procedure, and show that combining by meta-analytic methods avoids such problems.