Traditional narrative reviews have a number of disadvantages that systematic reviews may overcome. First, the classical review is subjective and therefore prone to bias and error.28 Mulrow showed that among 50 reviews published in the mid 1980s in leading general medicine journals, 49 reviews did not specify the source of the information and failed to perform a standardised assessment of the methodological quality of studies.1 Our junior doctor could have consulted another review of the same topic, published in the European Heart Journal in the same year. This review concluded that "it seems perfectly reasonable to treat patients who have survived an infarction with timolol".29 Without guidance by formal rules, reviewers will inevitably disagree about issues as basic as what types of studies it is appropriate to include and how to balance the quantitative evidence they provide. Selective inclusion of studies that support the author's view is common. This is illustrated by the observation that the frequency of citation of clinical trials is related to their outcome, with studies in line with the prevailing opinion being quoted more frequently than unsupportive studies30,31 Once a set of studies has been assembled a common way to review the results is to count the number of studies supporting various sides of an issue and to choose the view receiving the most votes. This procedure is clearly unsound, since it ignores sample size, effect size, and research design. It is thus hardly surprising that reviewers using traditional methods often reach opposite conclusions1 and miss small, but potentially important, differences.32 In controversial areas the conclusions drawn from a given body of evidence may be associated more with the speciality of the reviewer than with the available data.33 By systematically identifying, scrutinising, tabulating, and perhaps integrating all relevant studies, systematic reviews allow a more objective appraisal, which can help to resolve uncertainties when the original research, classical reviews and editorial comments disagree.
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