When searching for material for systematic reviews there is a tension between comprehensiveness (SENSITIVITY) and relevance (PRECISION). In general the more comprehensive the search the less relevant are the results. The following definitions are taken from The Cochrane Handbook for Systematic Reviews of Interventions:
Sensitivity = number of relevant reports identified
Total number of relevant reports in existence
Precision = number of relevant reports identified
Total number of reports identified
Developing a search strategy is an iterative process. Search terms are modified, based on what has already been retrieved. There are diminishing returns for search efforts; after a certain stage, each additional unit of time invested in searching returns fewer references that are relevant to the review. The decision as to how much to invest in the search process may depend on several factors, such as the question a review addresses, and the resources that are available.
Use the following freely available resources to search for systematic reviews on a range of subject areas:
When conducting a systematic review it is important to consider ALL relevant resources. These may include databases and grey literature, and the process may involve hand-searches of online and print materials, or citation chaining using the reference lists of key articles, depending on the question being asked and decisions you make around the scope of the review.
Bibliographic Databases - the term used to refer to digital collections of references to published literature. Good examples of bibliographic databases for biomedical subjects are Medline and Embase. When conducting a systematic review, it is important to select and search across a range of databases, as no single database can cover the range of relevant literature.
Grey Literature - the term used to describe research that is either unpublished or has been published in non-commercial form.
Handsearching - Handsearching is the task of manually searching through medical journals or conference abstract books for reports of controlled trials which are not indexed in the major electronic databases. For complete identification of reports of trials, electronic searching may need to be supplemented by conducting page by page searches of a variety of sources including journals and supplements, conference proceedings and abstracts, and correspondence.
Print Resources - not all relevant resources will be available electronically. When conducting a systematic review, the emphasis is on identifying and searching relevant sources, irrespective of format.
Citation chaining - the reference lists of key articles can be rich sources of further information on your topic. Citation chaining is when you use one source as a way to find related documents in a forwards or backwards process. With forward chaining you start with one work and then look at works that have cited that document. Backward chaining involves looking at a document to see what that document has cited. Citation chaining can help you find the root of an important idea or help you understand how an idea developed. You can also find related resources in your area of research.