Meta-analysis is a quantitative method of summarizing the results of several studies. One goal of meta-analysis is to estimate the size of the pooled association between a treatment or exposure and an outcome. This pooled association, or effect can be used to judge the clinical or public health impact of the exposure. A second goal is to seek evidence that the association varies according to the level of some other factor. This is often done using formal statistical tests of the hypothesis that the associations in the different studies are similar (homogeneous). A third goal is to estimate a variance so that the precision of the pooled association estimate may be judged using a confidence interval.