Systematic reviews - theory and practice

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Abstract
The consumer of health care services expects that medical practice is based on scientific evidence with regard to efficacy and effectiveness. Randomized controlled trials provide the most valid basis for comparison of interventions in health care and offer the most reliable information to guide clinical practice. The need for well-designed and performed systematic reviews of the available information to address a specific clinical question should be obvious both to the practicing clinician and the consumer. A systematic review is defined as “the application of scientific strategies that limits bias to the systematic assembly, critical appraisal, and synthesis of all relevant studies on a specific topic”. “Meta-analysis (quantitative overview) is a systematic review that employs statistical methods to combine and summarize the results of several trials”. In a meta-analysis the individual studies are weighted according to the inverse of the variance; that is more weight is given to studies with more events. The pooling of data allows for an increase in power and thus a more precise estimate of the effect size. Systematic reviews that meet explicit criteria for validity offer the reader information that as a rule is less biased than the unstructured overview which has traditionally been performed by one or several experts in a specific content area. Recently statistical methods have been developed to summarize data from studies of diagnostic tests. Cumulative meta-analyses offer the caregiver and the health-care consumer with answers regarding the effectiveness of a certain intervention at the earliest possible date in time.