ADVANTAGES OF META-ANALYSIS: Literature reviews have traditionally been largely narrative. Meta-analysis now offers the opportunity to critically evaluate and statistically combine results of comparable studies or trials. Its major purposes are to increase the numbers of observations and the statistical power, and to improve the estimates of the effect size of an intervention or an association.
Methods: There is, as yet, no unanimously accepted strategy for performing a meta-analysis but researchers agree that each meta-analysis should be conducted like a scientific experiment and begin with a protocol, which clearly states its aim and methodology. Meta-analysts disagree on the criteria for inclusion or exclusion of primary studies, with relation to publication status, comparability and required scientific quality, but sensitivity analyses make it possible to assess the impact of various selection criteria on the results. Several statistical methods have been developed to analyse data extracted from the literature; more recently, meta-analyses have also been performed on individual subject data.
Conclusions: Meta-analysis is superior to narrative reports for systematic reviews of the literature, but its quantitative results should be interpreted with caution even when the analysis is performed according to rigorous rules.