Systematic Reviews and Meta-Analyses
Systematic reviews and meta-analyses are methods of aggregating research. By synthetizing multiple studies, the intent is to provide an answer that is more comprehensive and precise (in the case of a meta-analysis) than can be provided by a single trial.
Systematic reviews begin with a transparent, systematic search for all potentially relevant studies addressing a well-defined research question. Only studies that meet the specified inclusion and exclusion criteria are included in the review. These studies are then evaluated for factors such as the risk of bias of individual studies and heterogeneity between studies. If the data is also combined quantitatively, it is also referred to as a meta-analysis.
While this can potentially lead to high-quality evidence, the review itself must be conducted properly. A systematic review can produce invalid results if the search systematically missed studies or if the research question was not sufficiently focused. To address such concerns, this chapter will provide guidance on thoroughly appraising a systematic review with or without meta-analysis.
This chapter is focused on systematic reviews and meta-analyses of RCTs, however these methods can also be applied to observational trials. Caution should be exercised with regards to such meta-analyses. This is because pooling observational trials will not reduce the potential biases and confounders found in the original studies. See Stroup DF et al. for a further discussion.
A review that systematically identifies all potentially relevant studies on a research question. The aggregate of studies is then evaluated with respect to factors such as risk of bias of individual studies or heterogeneity among results. The qualitative combination of results is a systematic review.
A meta-analysis is a quantitative combination of the data obtained in a systematic review.
Systematic deviation of an estimate from the truth (either an overestimation or underestimation) caused by a study design or conduct feature. See the Catalog of Bias for specific biases, explanations, and examples.
Refers to variability between studies in a systematic review. It can refer to clinical differences, methodological differences, or variable results between studies. Heterogeneity occurs on a continuum and, in the case of heterogeneity amongst results, can be expressed numerically via measures of statistical heterogeneity. See here for a further discussion of statistical heterogeneity.
Randomized controlled trials are those in which participants are randomly allocated to two or more groups which are given different treatments.
