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Systematic Reviews and Meta Analysis
A resource for finding data sources, filters, and standards to support systematic searches of the biomedical literature.
There are dozens of filters available for limiting searches to randomized controlled trials . One of the following filters should be adequate for most investigations. Red is for PubMed; Blue is for Embase.
The NCBI's Clinical Queries for Therapy (sensitive) is based on work of Haynes, et al.  and provides high sensitivty (99%) at the expense of precision (10%).
(clinical[tiab] AND trial[tiab]) OR "clinical trials as topic"[mesh] OR "clinical trial"[pt] OR random*[tiab] OR "random allocation"[mesh] OR "therapeutic use"[sh]
('clinical':ti,ab AND 'trial':ti,ab) OR 'clinical trial'/exp OR random* OR 'drug therapy':lnk
The Cochrane Collaboration publishes an extensive handbook for systematic reviews. Section 6.4.11  of this Handbook includes strategies for limiting searches to controlled trials. The "Highly Sensitive Search Strategy" has been used widely to retrieve randomized controlled trials and other types of evidence for systematic reviews. Sensitivity, 98%; precision, 13% .
"randomized controlled trial"[pt] OR "controlled clinical trial"[pt] OR randomized[tiab] OR placebo[tiab] OR "drug therapy"[sh] OR randomly[tiab] OR trial[tiab] OR groups[tiab]
'randomized controlled trial'/exp OR 'controlled clinical trial'/exp OR randomized:ti,ab OR placebo:ti,ab OR 'drug therapy':lnk OR randomly:ti,ab OR trial:ti,ab OR groups:ti,ab
As an alternative, Royle and Waugh  published 'simplified' strategy that is nearly as effective as the foregoing methods (Sensitivity, 97%; precision, 29% ), induces little or no bias, and is easy to transfer to interfaces other than PubMed. It is:
For precise limiting when you're pressed for time, you can use the randomized controlled trial limits in either PubMed or Embase. Used as limits or their underlying controlled vocabulary terms, they've been reported to produce extremely high precision (94%!) with only a small sacrifice in sensitivity . The terms from MeSH or Emtree are:
"randomized controlled trial"[pt]
'randomized controlled trial'/exp
However, the Cochrane Collaboration ceased tagging MEDLINE records in 2006. It's been observed that many RCTs are now missed by the RCT publication type .
1. McKibbon KA, Wilczynski NL, Haynes RB; Hedges Team. Retrieving randomized controlled trials from medline: a comparison of 38 published search filters. Health Info Libr J. 2009 Sep;26(3):187-202. PubMed PMID: 19712211.
2. Haynes RB, McKibbon KA, Wilczynski NL, Walter SD, Werre SR; Hedges Team. Optimal search strategies for retrieving scientifically strong studies of treatment from Medline: analytical survey. BMJ. 2005 May 21;330(7501):1179. PubMed PMID: 15894554.
4. Royle PL, Waugh NR. A simplified search strategy for identifying randomised controlled trials for systematic reviews of health care interventions: a comparison with more exhaustive strategies. BMC Med Res Methodol. 2005 Jul 23;5:23. PubMed PMID: 16042789.
5. Royle PL, Waugh NR. Making literature searches easier: a rapid and sensitive search filter for retrieving randomized controlled trials from PubMed. Diabet Med. 2007 Mar;24(3):308-11. Review. PubMed PMID: 17305790.
6. Wieland LS, Robinson KA, Dickersin K. Understanding why evidence from randomised clinical trials may not be retrieved from Medline: comparison of indexed and non-indexed records. BMJ. 2012 Jan 3;344:d7501. doi:10.1136/bmj.d7501. PubMed PMID: 22214757.