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What is a systematic review?
  1. Jane Clarke
  1. Correspondence to Jane Clarke
    4 Prime Road, Grey Lynn, Auckland, New Zealand; janeclarkehome{at}

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A high-quality systematic review is described as the most reliable source of evidence to guide clinical practice. The purpose of a systematic review is to deliver a meticulous summary of all the available primary research in response to a research question. A systematic review uses all the existing research and is sometime called ‘secondary research’ (research on research). They are often required by research funders to establish the state of existing knowledge and are frequently used in guideline development. Systematic review findings are often used within the healthcare setting but may be applied elsewhere. For example, the Campbell Collaboration advocates the application of systematic reviews for policy-making in education, justice and social work.

Systematic reviews can be conducted on all types of primary research. Many are reviews of randomised trials (addressing questions of effectiveness), cross-sectional studies (addressing questions about prevalence or diagnostic accuracy, for example) or cohort studies (addressing questions about prognosis). When qualitative research is reviewed systematically, it may be described as a systematic review, but more often other terms such as meta-synthesis are used.

Systematic review methodology is explicit and precise and aims to minimise bias, thus enhancing the reliability of the conclusions drawn.1 2 The features of a systematic review include:

  • clear aims with predetermined eligibility and relevance criteria for studies;

  • transparent, reproducible methods;

  • rigorous search designed to locate all eligible studies;

  • an assessment of the validity of the findings of the included studies and

  • a systematic presentation, and synthesis, of the included studies.3

The first step in a systematic review is a meticulous search of all sources of evidence for relevant studies. The databases and citation indexes searched are listed in the methodology section of the review. Next, using predetermined reproducible criteria to screen for eligibility and relevance assessment of titles and the abstracts is completed. Each study is then assessed in terms of methodological quality.

Finally, the evidence is synthesised. This process may or may not include a meta-analysis. A meta-analysis is a statistical summary of the findings of independent studies.4 Meta-analyses can potentially present more precise estimates of the effects of interventions than those derived from the individual studies alone. These strategies are used to limit bias and random error which may arise during this process. Without these safeguards, then, reviews can mislead, such that we gain an unreliable summary of the available knowledge.

The Cochrane Collaboration is a leader in the production of systematic reviews. Cochrane reviews are published on a monthly basis in the Cochrane Database of Systematic Reviews in The Cochrane Library (see:

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  • Competing interests None.

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