Application of meta-analysis in reviewing occupational cohort studies.

Meta-analysis has been used increasingly in reviewing and summarising epidemiological studies. Reviews incorporating meta-analyses have appeared in medical journals in increasing numbers. Although there are several methodology papers on meta-analysis, most of these papers have been written primarily for discussion among epidemiologists. The present paper considers some of the basic methodological issues, the more practical aspects of meta-analysis, and targets an audience of mainly non-epidemiologists. Thus, the main objective of this paper is to provide some basic guidelines for non-epidemiologists to evaluate meta-analysis in occupational cohort studies. In this methodology paper, the limitations and problems of traditional qualitative reviews are pointed out. Some of these problems can be dealt with by quantitative meta-analysis. The potential limitations and benefits of quantitative meta-analysis are discussed. Rather than replacing traditional qualitative review, quantitative meta-analysis should be made part of the overall assessment. The term "meta-review" is proposed to emphasise the importance of both qualitative and quantitative components in a comprehensive review process. The basic steps in a meta-review are outlined, with a discussion on how to recognise and avoid some of the problems which are likely to occur at each step. A meta-review is useful in selecting studies, and in organising, presenting, and summarising results from individual studies. A meta-review can also be used to detect heterogeneity among studies. Major benefits of conducting a meta-analysis (the quantitative component in a meta-review) include the increase in statistical power and the estimate of a properly weighted summary risk estimate.