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It's about integrating individual clinical expertise and the best external evidence. Good doctors use both individual clinical expertise and the best available external evidence (laboratory support, published papers etc), and neither alone is enough. Evidence based medicine is not restricted to randomized trials and meta-analyses. It involves tracking down the best external evidence with which to answer our clinical questions. To find out about the accuracy of a diagnostic test, we need to find proper cross sectional studies of patients clinically suspected of harbouring the relevant disorder, not a randomised trial. For a question about prognosis, we need proper follow up studies of patients assembled at a uniform, early point in the clinical course of their disease. And sometimes the evidence we need will come from the basic sciences such as genetics or immunology. It is when asking questions about therapy that we should try to avoid the non-experimental approaches, since these routinely lead to false positive conclusions about efficacy. Because the randomised trial, and especially the systematic review of several randomised trials, is so much more likely to inform us and so much less likely to mislead us, it has become the "gold standard" for judging whether a treatment does more good than harm. However, some questions about therapy do not require randomised trials (successful interventions for otherwise fatal conditions) or cannot wait for the trials to be conducted. And if no randomised trial has been carried out for our patient's predicament, we must follow the trail to the next best external evidence and work from there.

Why conduct a systematic review?

  1. To answer questions where the answer is uncertain
  2. To explain variations in practice
  3. To confirm the appropriateness of current practice
  4. To summarize and help people to understand the evidence

Steps of systematic review

  1. Define the question
  2. Look for all studies reliably addressing the question
  3. Examine the studies closely to select relevant ones
  4. Assess the quality of the studies
  5. Calculate results for each study (and combine them if appropriate). A systematic review may have a statistical combination of studies (a meta-analysis) but not necessary always
  6. Interpret results

Rationale for systematic reviews

  1. Large quantities of information can be reduced into palatable pieces for digestion using systematic reviews. Over two million articles are published annually in the biomedical literature in over 20,000 journals. Systematic literature review is needed to refine these unmanageable amounts of information. Other reasons to conduct systematic reviews include
  2. Increased power and precision: An example of meta-analysis improving statistical power is shown in the Cochrane Collaboration's logo, which depicts effect sizes of seven trials that evaluated the effects of a short course of corticosteroids given to women expected to give birth prematurely. Only two trials had clear cut, statistically significant effects, but when data from all of the studies were pooled the "sample size" and thus power increased, yielding a definitive significant combined effect size that indicated strongly that corticosteroids reduce the risk of babies dying from complications of immaturity . Unique advantages of quantitative systematic reviews or meta-analyses are increased power and precision in estimating effects and risks particularly relevant to the needs of drug manufacturers.
  3. The generalisability of scientific findings can be established in systematic reviews. Consistency of treatment effects across different disease with common underlying pathophysiology and consistency of risk factors across study populations can be ascertained
  4. Decision makers need to integrate the critical pieces of available biomedical information. Systematic reviews are used by more specialized integrators, such as economic and decision analysts, to estimate the variables and outcomes that are included in their evaluations. Health policy makers in the West use systematic reviews to formulate guidelines and legislation concerning the use of certain diagnostic tests and treatment strategies.


David L Sackett, William M C Rosenberg, J A Muir Gray, R Brian Haynes, W Scott Richardson. BMJ 1996;312:71-72

C D Mulrow. BMJ 1994; 309: 597-599

Ad Hoc Working Group for Critical Appraisal of the Medical Literature. Academia and clinic: a proposal for more informative abstracts of clinical articles. Ann Intern Med 1987; 106: 598-604



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