- Trisha Greenhalgh, senior lecturer (firstname.lastname@example.org)a
- a Unit for Evidence-Based Practice and Policy, Department of Primary Care and Population Sciences, University College London Medical School/Royal Free Hospital School of Medicine, Whittington Hospital, London N19 5NF
Remember the essays you used to write as a student? You would browse through the indexes of books and journals until you came across a paragraph that looked relevant, and copied it out. If anything you found did not fit in with the theory you were proposing, you left it out. This, more or less, constitutes the methodology of the journalistic review—an overview of primary studies which have not been identified or analysed in a systematic (standardised and objective) way.
A systematic review is an overview of primary studies that used explicit and reproducible methods
A meta-analysis is a mathematical synthesis of the results of two or more primary studies that addressed the same hypothesis in the same way
Although meta-analysis can increase the precision of a result, it is important to ensure that the methods used for the review were valid and reliable
In contrast, a systematic review is an overview of primary studies which contains an explicit statement of objectives, materials, and methods and has been conducted according to explicit and reproducible methodology (fig 1).
Some advantages of the systematic review are given in box. When a systematic review is undertaken, not only must the search for relevant articles be thorough and objective, but the criteria used to reject articles as “flawed” must be explicit and independent of the results of those trials. The most enduring and useful systematic reviews, notably those undertaken by the Cochrane Collaboration, are regularly updated to incorporate new evidence.2
Box 1: Advantages of systematic reviews3
Explicit methods limit bias in identifying and rejecting studies
Conclusions are more reliable and accurate because of methods used
Large amounts of information can be assimilated quickly by healthcare providers, researchers, and policymakers
Delay between research discoveries and implementation of effective …