When preparing systematic reviews, ideally we try to find every study that's ever been accomplished addressing our review's question. But this goal is hard to achieve, and practically impossible in many cases. Many studies are never published, and of those which are published, many are not indexed by well known databases, thus too much effort is needed for identifying studies.
Because of their most reliable estimates of effects, systematic reviews generally focus on reports from Randomized Controlled Trials (RCTs), when such data are available. Studies have shown that only about half of all known published RCTs were identifiable using MEDLINE; As a consequence, a search in MEDLINE is generally not considered adequate for preparing a system review. A comprehensive search for relevant RCTs, which seeks to minimize bias, is one of the essential steps in doing a systematic review and one of the factors that distinguishes a systematic review from a traditional review.

Sources for searching:

1. Electronic databases:
   Such as MEDLINE, EMBASE, Cochrane Library, etc… (We offer a Clinical Trials Registries Database, which contains links to online registries of ongoing and completed clinical trials.)
2. Hand searching:
   For identifying published studies which are not electronically available.
3. Identifying unpublished studies:
   Using gray literature in preparing systematic reviews is mandatory to overcome or alleviate publication bias. Gray literature includes conference proceedings, dissertations, theses, clinical trials registries and other reports.
4. Checking reference lists:
   This is to follow up references from one article to another including those from previously published systematic reviews, to identify relevant reports. The process of following up references from one article to another is generally an efficient means of identifying studies for possible inclusion in a review.
5. Personal communication:
   To know of studies you not found in the previous steps, reviewers can send a list of the studies they have found to the authors of those studies or pharmaceutical agent, asking if they are aware of any other relevant studies (published or unpublished).

Search strategy:

Developing and documenting search strategy and keeping track of identified studies is necessary in systematic reviews.
Balance between comprehensiveness and precision when developing a search strategy is necessary. Developing a search strategy is a continuous process in which terms that are used are modified, based on what has already been retrieved.
Documenting search strategy means knowing that how the search is done, what is searched for and when the search is done for each database.
In order to preventing duplication of inclusion of single study in analysis of results, we need some system for keeping track of which references that we think are relevant.

Our Clinical Trials Registries Database which is updated every often, contains a list of all Clinical Trials Registries known to us. Don't hesitate to contact us to let us know of more of such online registers.

References and links:

1. Cochrane Handbook for Systematic Reviews of Interventions
2. Searchable database of research papers
3. Index to theses
4. Current Contents Connect

Kazem Heidari Tavasani

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