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Systematic Reviews

All about systematic reviews: what they are, their history, types, how to read them, how to conduct them, and how to get help with them from the library.


If you are new to the world of systematic reviews, you may find it useful context to understand how they originated as a research method, why they are conducted, and how their production has grown since they were originally proposed. Systematic reviews have their origins in randomized controlled trials and have overtaken them in number in recent years.

Before Systematic Reviews: The Advent of Randomized Controlled Trials (RCTs)

Photograph of Dr. Archie Cochrane

Dr. Archie Cochrane, CBE is considered one of the founders of Clinical Epidemiology and the Evidence-based Practice movement. In his 1972 book, Effectiveness and Efficiency: Random Reflections on Health Services, (Cochrane AL. Effectiveness and Efficiency: Random Reflections on Health Services. London: Nuffield Provincial Hospitals Trust; 1972.) he advocated for the use of Randomized Controlled Trials to improve the quality of medical practice.

Image of Dr. Archie Cochrane from Cardiff University Library, Cochrane Archive, University Hospital Llandough and Archie Cochrane: the name behind Cochrane.

The Advent of Systematic Reviews

In 1979, MEDLINE added over 282,000 records, of which 1,786 were clinical trials. Cochrane was concerned that practitioners were being overwhelmed by the increasing number of RCTs being published and of the growing lack of consistency between their recommendations. That year he wrote the following statement:

“It is surely a great criticism of our profession that we have not organised a critical summary, by speciality and subspeciality, adapted periodically, of all randomised controlled trials”.

Cochrane AL (1979). 1931-1971: a critical review, with particular reference to the medical profession. In: Teeling-Smith G, ed. Medicines for the Year 2000. London: Office of Health Economics, pp 1-11.

With this statement, Cochrane is advocating for the creation of a new publication type that we know today as the systematic review. With the creation of the Cochrane Collaboration in 1993, The status of Systematic Reviews as a cornerstone of the Evidence-based Practice process was firmly established.

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Systematic Reviews Today

  • The yearly number of systematic reviews being published, including scoping reviews, rapid reviews, and other types of systematic reviews, has been steadily rising over the last 25 years
  • In 2019, the annual number of systematic reviews added to PubMed surpassed that seen for controlled clinical trials, and has continued to rise while clinical trials have remained relatively flat since peaking in 2013
  • In 2022, 45,533 systematic reviews were added to PubMed, which is equal to almost 125 per day

Chart showng number of Randomized Controlled Trials (RCTs) vs Systematic Reveiws (SRs) added to PubMed from 2000-2022. RCTs peak in 2013 and roughly plateau after. SRs increase steadily until 2019, when they increase more sharply and overtake RCTs, to the point where 1.5 SRs are added to PubMed for each RCT in 2022.

Randomized Controlled Trials (RCTs)/Controlled Clinical Trials (CCTs) vs Systematic Reviews/Meta-Analyses added to PubMed annually, 2000-2022
Year "Randomized Controlled Trial" [Publication Type] OR "Controlled Clinical Trial" [Publication Type] "Systematic Review" [Publication Type] OR "Meta-Analysis" [Publication Type]
2000 13,648 2,161


2002 14,468 2,041
2003 16,168 2,314
2004 18,321 2,818
2005 19,351 3,380
2006 19,992 4,155
2007 21,318 5,163
2008 21,885 5,590
2009 22,737 6,714
2010 24,487 8,148
2011 27,409 10,251
2012 29,813 12,983
2013 32,075 15,910
2014 31,985 18,530
2015 31,363 20,521
2016 29,995 22,112
2017 29,551 24,172
2018 28,094 26,218
2019 27,992 30,178
2020 29,756 36,647
2021 30,474 43,339
2022 29,382


2023 23,871 37,611

Note: Data as of December 2023. Results returned by linked PubMed searches may vary from numbers shown here due to changes made to the database to remove records that are duplicate or out-of-scope.

Additional Information