Searches for evidence
As a systematic review aims to collect all available evidence on a specific topic, extensive searches are required. A limited search may find an unrepresentative set of studies, possibly leading to incomplete results, selection bias, and reduced generalisability of the results of a systematic review. In order to identify as many relevant studies as possible, usually multiple sources are searched for a systematic review. These are not only the major bibliographic databases like MEDLINE and Embase, but also other databases like relevant subject specific databases (e.g. Cumulative Index to Nursing and Allied Health Literature [CINAHL] and PsychINFO [psychology and related fields]) or regional databases (e.g. LILACS for Latin American and the Caribbean). In addition, also other sources should be searched to identify relevant evidence, such as reference lists, conference proceedings, trial registries, economic databases, websites, and personal communication with authors or experts in the field.
A Cochrane review on neuraminidase inhibitors for preventing and treating influenza in adults and children is an example of the importance of going beyond the regular bibliographic databases when searching for a systematic review (Jefferson 2014). After assessing regulatory documents in addition to published trials, the review confirms small benefits on symptom relief, however, little evidence was found to support previous beliefs of any beneficial effect on hospital admissions or serious complications.