Systematic reviews

The form of a systematic review encourages the introduction of basic epidemiological principles and quantification into the process of reviewing. Gene Glass, an educational psychologist, was the first to add the results of similar studies in the hope of quantifying the effects of a treatment. (1...2> Glass defined 'meta-analysis' as 'the statistical analysis of a large collection of analyses results from individual studies for the purpose of integrating the findings'. (1.3) Unsurprisingly, in the sensitive area of the psychotherapies, their first and flawed attempts in the new discipline generated controversy. (14) Critics were quick to point out that drawing conclusions from summation of very different types of therapies, undertaken by practitioners of varied experience, was likely to be inadvisable. These pioneers, who even years later are still being criticized for adding 'apples and oranges', (1.9 are nevertheless owed a great debt by the rest of medicine. It has taken nearly two decades for those supporting this process of quantification to suggest that adding apples and oranges may not be inadvisable if fruit pie is the desired outcome. (16,>

Systematic reviews attempt to minimize bias in the identification, extraction, and summation of relevant data by applying good survey methods to the process of literature reviewing. An analogy may help. In a community survey of the prevalence of mental disorders a researcher stands on the doorstep of the hospital and suggests that 5 per cent of the population suffers from serious mental illness. By chance, the final estimate may even be correct, but the work could not be seen as methodologically rigorous. The researcher should have written a study protocol, clearly defined an unbiased sample of individuals to interview, and specified a priori the analyses to be undertaken. A systematic review should do this for a survey of a 'population' of relevant literature. Within such a review, the objectives, criteria for selection of relevant studies, search strategy, methods of study selection, data extraction, and assimilation are all made explicit.

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