Time frame
Language
Source type and focus
Articles? Books? Only peer-reviewed? Review articles? Grey literature?
Reduce your research question to terms and concepts
Start broad, get narrower as keywords develop
Keep track of what searches work and what don’t
Citation indexes: Web of Science and Scopus; PubMed
Publisher databases: ScienceDirect, SpringerLink, Wiley Online Library, etc.
Sources considered gray — data collected by a federal agency, or a technical report by a professional association, for example — are certainly edited in some way but do not go through peer review, so while they are published documents, they are not considered scholarly articles.