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Systematic Reviews

An introduction to systematic reviews, with examples from health sciences and medicine

Systematic Reviews

Systematic Reviews: The Search Process


Step 1 : Develop a Research Question and Review Approach

  • Does the question need answers, a decision or the best estimate?
  • If it needs to be fast and authoritative, conduct a rapid review from the best resources.
  • Scoping reviews – or a systematic review – take more time and aim to cover more or all available resources.
  • What will be an acceptable answer?
  • Is <this> intervention with <these> patients more likely to give <these> outputs than another intervention?

Step 2 : Identify Resources and Build a Search Strategy

  • Use resources selected for coverage and reputation.
  • Every question has angles and contexts.
  • Search with a distinct set of words - concepts - targeting each of the main facets of the question.
  • Adapt the strategy for searching in each resource.
  • Record when, where and how you searched and the results.
  • Save, identify and deduplicate the results for critical appraisal.

Step 3 : Analyse and Report the Results

  • The search strategy will not have filtered out all irrelevant results. Refine  the strategy.  Apply exclusion criteria.
  • Screen out inappropriate studies using exclusion criteria, recording what is  excluded and why.
  • A review matrix and summary tables can help with compiling results.
  • A forest plot can visualise statistical results.


A systematic review has a number of features distinguishing it from merely searching systematically.

It is:

  • comprehensive
  • objective
  • transparent
  • fully documented and reproducible
  • conducted across a range of resources

It aims to identify as many relevant studies as possible, even if some will be later excluded on technical grounds. 

It is also

  • time-consuming (it may take 8 hours or more),
  • rigorous and exacting (it maps to subject headings and synonyms to be as comprehensive as possible)
  • and it rates wide recall over specificity (it is sensitive to borderline studies, which may need closer examination in terms of inclusion and exclusion criteria).

The inclusion and exclusion criteria may be based on participant recruitment and follow-up, randomisation, sampling or other details of methodology, and validity. In critical appraisal the significance and generalizability of findings are also assessed.

You might not need a systematic review. It's a scholarly research project usually undertaken by experts. 

If you need an authoritative source summarising the evidence available, a Cochrane review or clinical guidelines may be more appropriate - or perhaps you could conduct a rapid review.

If you're unsure of the extent of the literature, or still finalising the inclusion and exclusion criteria, a scoping review is indicated.

Rapid reviews target high quality and authoritative resources for time-critical decision-making or clinically urgent questions.

Yet like a systematic review they aim to identify the key concepts, theories and resources in a field, and to survey the major research studies. 

Less time may be spent on critical appraisal as systematic reviews, evidence briefs and clinical guidelines are sought in preference to exhaustive coverage of primary studies.


A scoping review is carried out when little is known in advance about the extent and consistency of the literature in a field, or as a preliminary to a systematic review.

As with systematic reviews and rapid reviews, they aim to identify the key concepts, theories and resources in a field, and to survey the findings of the major research studies. However they are usually a preliminary mapping of resources for a subsequent full systematic review rather than an end in themselves.

They may be indicative rather than exhaustive, and indicate likely sources of resources such as grey literature. They are also more likely to be iterative and organic in their development whereas a systematic review  has pre-determined inclusion and exclusion criteria.

See for example

Peters, M. D., Godfrey, C. M., Khalil, H., McInerney, P., Parker, D., & Soares, C. B. (2015). Guidance for conducting systematic scoping reviews.International journal of evidence-based healthcare, 13(3), 141-146.

Peterson, J., Pearce, P. F., Ferguson, L. A., & Langford, C. A. (2016). Understanding scoping reviews: Definition, purpose, and process. Journal of the American Association of Nurse Practitioners.

Yost, J., Thompson, D., Ganann, R., Aloweni, F., Newman, K., McKibbon, A., ... & Ciliska, D. (2014). Knowledge Translation Strategies for Enhancing Nurses’ Evidenceā€Informed Decision Making: A Scoping Review. Worldviews on Evidenceā€Based Nursing, 11(3), 156-167.