Identifying trials

Systematic reviews should attempt to locate information on all pertinent trials and will almost always need to go beyond a simple search ofbibliographic databases for relevant publications. Not only are positive trials more likely to be published than negative or inconclusive ones, but particularly striking results are perhaps more likely to be published in those English language [38] high-impact journals indexed by bibliographic databases. Furthermore, such databases do not cover all medical journals - MEDLINE, for example, indexes less than 4000 out of around 16,000 biomedical journals and EMBASE indexes 4000.

Systematic reviews that restrict trial identification to searches of bibliographic databases and exclude unpublished trials, trials published in non-English language journals, or those reported in the 'grey' literature (meeting abstracts, theses, etc.) can lead to exaggerated estimates of intervention effectiveness [39-41]. It is therefore important to employ a well thought out strategy for identifying trials. Failure to do so is likely to result in only a proportion of all eligible trials being identified (Fig. 11.4) and could bias the results of any systematic review. Box 11.3 lists various ways to search for trials depending on the question posed and the type of systematic review planned.

The Cochrane Collaboration has been responsible for a number of initiatives to make it easier for reviewers to identify RCTs. Since 1994 it has contributed to the retrospective re-tagging and indexing of 100,000 trials that were not previously indexed appropriately as RCTs in MEDLINE and has also been involved in a project to identify and re-tag trials within EMBASE. It is also responsible for compiling the Cochrane Controlled Trials Register (CENTRAL) that contains details of and citations to all reports of controlled trials that have been identified through its extensive systematic searches of the world medical literature, including searches of MEDLINE, EMBASE and an extensive programme of handsearching. CENTRAL is already the largest single source of RCT

Hand Searches (3)

Word of Mouth (3)

Trial Registers (3)

Hand Searches (3)

Word of Mouth (3)

Trial Registers (3)

MEDLINE/CANCERLIT(12)

Fig. 11.4 Methods of identifying trials at initiation of a meta-analysis of neo-adjuvant chemotherapy for cancer of the uterine cervix [42].

MEDLINE/CANCERLIT(12)

Fig. 11.4 Methods of identifying trials at initiation of a meta-analysis of neo-adjuvant chemotherapy for cancer of the uterine cervix [42].

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