Systematic literature reviews including meta-analyses are invaluable scientific activities. The rationale for such reviews is well established. Health care providers, researchers, and policy makers are inundated with unmanageable amounts of information; they need systematic reviews to efficiently integrate existing information and provide data for rational decision making. Systematic reviews establish whether scientific findings are consistent and can be generalised across populations, settings, and treatment variations, or whether findings vary significantly by particular subsets. Meta-analyses in particular can increase power and precision of estimates of treatment effects and exposure risks. Finally explicit methods used in systematic reviews limit bias and, hopefully, will improve reliability and accuracy of conclusions.

Systematic literature review is a fundamental scientific activity. Its rationale is grounded firmly in several premises. Firstly, large quantities of information must be reduced into palatable pieces for digestion. Over two million articles are published annually in the biomedical literature in over 20 000 journals 1-literally a small mountain of information. For example, about 4400 pages were devoted to approximately 1100 articles in the BMJ and New England Journal of Medicine, combined, in 1992. In a stack, two million such articles would rise 500 m. Clearly, systematic literature review is needed to refine these unmanageable amounts of information. Through critical exploration, evaluation, and synthesis the systematic review separates the insignificant, unsound, or redundant deadwood in the medical literature from the salient and critical studies that are worthy of reflection. 2