Meta-analysis is the statistical synthesis of the data from a set of comparable studies of a problem, and it yields a quantitative summary of the pooled results. It is the process of aggregating the data and results of a set of studies, preferably as many as possible that have used the same or similar methods and procedures; reanalyzing the data from all these combined studies; and thereby generating larger numbers and more stable rates and proportions for statistical analysis and significance testing than can be achieved by any single study. The process is widely used in the biomedical sciences, especially in epidemiology and in clinical trials. In these applications, meta-analysis is defined as the systematic, organized, and structured evaluation of a problem of interest. The essence of the process is the use of statistical tables or similar data from previously published peer-reviewed and independently conducted studies of a particular problem. It is most commonly used to assemble the findings from a series of randomized controlled trials, none of which on its own would necessarily have sufficient statistical power to demonstrate statistically significant findings. The aggregated results, however, are capable of generating meaningful and statistically significant results.
There are some essential prerequisites for meta-analysis to be valid. Qualitatively, all studies included in a meta-analysis must fulfill predetermined criteria. All must have used essentially the same or closely comparable methods and procedures; the populations studied must be comparable; and the data must be complete and free of biases—such as those due to selection or exclusion criteria. Quantitatively, the raw data from all studies is usually reanalyzed, partly to verify the original findings from these studies, and partly to provide a database for summative analysis of the entire set of data. All eligible studies must be included in the meta-analysis. If a conscious decision is made to exclude some, there is always a suspicion that this was done in order to achieve a desired result. If a pharmaceutical or other commercial organization conducts a meta-analysis of studies aimed at showing its product in a favorable
A variation of the concept is a systematic review, defined as the application of strategies that limit bias in the assembly, critical appraisal, and synthesis of all relevant studies of a specific topic. Meta-analysis may be, but is not necessarily, used as part of this process. Systematic reviews are conducted on peer-reviewed publications dealing with a particular health problem and use rigorous, standardized methods for the selection and assessment of these publications. A systematic review can be conducted on observational (case-control or cohort) studies as well as on randomized controlled trials.
JOHN M. LAST
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