There are two key types of sources to search - licensed databases and alternate sources. Techniques for finding alternate sources include searching sources for grey literature; backward and forward reference chaining; hand searching; and contacting experts in the field.
We recommend Ovid Medline, rather than PubMed, for systematic reviews for several reasons. Ovid Medline allows you to:
Some systematic reviews state in their methods section that they've searched both Medline and PubMed. However, there's nearly complete overlap between Ovid Medline and PubMed so there's generally not a need to search them independently.
Should you prefer to search PubMed for your systematic review, consider contacting your subject librarian for help devising your search strategy.
Grey literature refers to materials that aren't published in a traditional way, including dissertations, conference proceedings, clinical trials, and clinical practice guidelines. These sources aren't usually covered in regular licensed databases. Below are some tools for finding grey literature.
Check out the citations in the reference lists of major studies. Examine what later studies have cited significant studies in these lists by searching:
Handsearching means manually looking through journals of interest to check that nothing was missed when searching licensed databases. For your topic, ensure that key journals are either adequately indexed in the databases you're searching, and if not, consider handsearching (ask your librarian if you need help determining where a journal is indexed).
Another idea for uncovering unpublished research is to identify key researchers working on your topic of interest to identify studies in process. For instance, you might be able to get additional data on a study described in a clinical trials register or conference abstract, or on a review protocol.