Health care professionals practicing evidence-based care, and librarians and information specialists supporting them, need easy access to the best evidence. Access to best evidence from systematic reviews, meta-analyses, and other summaries of the evidence is improving all the time, through the international efforts of The Cochrane Collaboration through the publication of The Cochrane Library [1
], health technology assessment programs such as those of the Agency for Healthcare Research and Quality [2
], the BMJ's Clinical Evidence [3
], the ACP Journal Club [4
], and the work of many others. Well-designed and conducted randomized controlled trials (RCTs) often form the most reliable input to systematic reviews and meta-analyses of health care interventions and, where practical and ethical, can provide the best evidence in the absence of systematic reviews. Access to RCTs has improved greatly since problems with identifying trials in MEDLINE were presented at a meeting at the US National Institutes of Health in 1993 [5
] and subsequently reported in a systematic review by Dickersin and colleagues in the BMJ
in 1994 [6
Dickersin and colleagues suggested improvements in the ways that authors should describe their work and recommended better indexing of RCTs in MEDLINE and changes to the methods used to identify trials in MEDLINE [6
]. The paper included a highly sensitive search strategy, designed by one of the authors (Lefebvre), to identify RCTs in MEDLINE. This subsequently became known as the Cochrane Highly Sensitive Search Strategy (Cochrane HSSS). This paper represented important information for researchers conducting systematic reviews, for clinicians trying to find best evidence from RCTs, and for librarians and information specialists supporting them in identifying the RCTs. The Cochrane HSSS has been used extensively over the last ten years by those involved in preparing systematic reviews, meta-analyses, and health technology assessment reports and has been recommended in guidelines for Cochrane reviewers [7, 8
However, databases change over time in content, indexing practice, and other features. The last ten years have seen several developments that follow Dickersin and colleagues' recommendations. First, the US National Library of Medicine (NLM) applied indexing improvements to MEDLINE, as a result of the paper presented to the National Institutes of Health in December 1993 [5
]. A higher proportion of RCTs were subsequently correctly indexed with the Publication Type, “Randomized Controlled Trial” [9, 10
]. In addition, a new Publication Type, “Controlled Clinical Trial,” was introduced in 1995. Secondly, NLM announced that they would “retag” reports of RCTs not already indexed with the appropriate “Randomized Controlled Trial” or “Clinical Controlled Trial” Publication Types.
As a consequence, The Cochrane Collaboration embarked on an extensive program to identify “untagged” RCTs in MEDLINE by reading the titles and abstracts of candidate records, published both before and after the introduction of the “Randomized Controlled Trial” (1991) and “Clinical Controlled Trial” (1995) Publication Types. Identified trial reports that are not already indexed with the “Randomized Controlled Trial” or “Clinical Controlled Trial” Publication Types are forwarded to NLM for retagging in MEDLINE [9
]. Over the last 10 years, the number of RCTs in humans indexed with the appropriate Publication Types, and, therefore, easily and accurately identifiable in MEDLINE, has risen from 20,000 (1993) to more than 270,000 in October 2005 (of which 100,000 were published before 1993). Retagging of MEDLINE continues annually and is usually a year behind the current MEDLINE publication year.
Beyond improvements to MEDLINE indexing, RCTs have also become more accessible through The Cochrane Collaboration's Cochrane Central Register of Controlled Trials (CENTRAL), published and updated quarterly in The Cochrane Library [1
]. CENTRAL contains 455,000 records of RCTs and clinical controlled trials (CCTs) as of issue 3 in 2005. All MEDLINE records relevant to humans with the “Randomized Controlled Trial” or “Clinical Controlled Trial” Publication Types are included, along with many thousands of non-MEDLINE records identified from The Cochrane Collaboration's journal hand-searching program and databases such as EMBASE [11
In addition to NLM's concentration on correctly identifying and indexing RCTs in MEDLINE, authors and journal editors are paying increased attention to reporting research methods clearly in the titles and abstracts of reports of RCTs in journals. The Consolidated Standards of Reporting Trials (CONSORT) statement gives clear guidelines on describing methods [12
], and positive effects of these recommendations have already been reported [13, 14
]. This should mean that database indexers are more likely to identify and correctly index reports of RCTs. These major efforts mean that ten years on from its publication in the Dickersin BMJ
], the long and complex Cochrane HSSS might no longer be the most efficient tool for librarians and other searchers and should be reassessed to identify whether alternative search terms might be more efficient.
The methods of search strategy design have also developed with a move toward more objective and research-based approaches. The Cochrane HSSS was based on a subjective selection of search terms compiled from free-text terms recommended by individuals with expertise in clinical trials and experience in searching for them in MEDLINE, together with Medical Subject Headings (MeSH) identified by one of the authors (Lefebvre). It was not derived from, or tested on, a gold standard of known reports of RCTs [6
]. Haynes and colleagues used subjectively derived search terms to identify “clinically sound” studies of therapy, which focused on identifying RCTs but tested strategy performance on gold standard sets of known records [15
]. Their strategy, updated in February 2004, has been included in the PubMed version of MEDLINE as a time-saving feature for searchers (Clinical Queries) [16
In recent years, researchers have developed search strategy design further to improve the objectivity of their methods. Techniques such as word frequency analysis and discriminant analysis have been used to derive objectively, through statistical analysis, the most efficient search terms to find desired types of records. Methods that the Centre for Reviews and Dissemination (CRD) and UK Cochrane Centre (UKCC) Search Filters Design Group developed and refined [17,18
] form the basis of this current research to develop efficient search strategies for busy clinicians who wish to identify RCTs in MEDLINE reliably and for systematic reviewers, meta-analysts, and others requiring higher sensitivity but with acceptable precision—and for librarians and information specialists who support these activities.