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Search for one of your diseases as a keyword. See if there are any subjects in the search that correlate with your keywords.
Use quotes when searching phrases.
Do not search full sentences - leave out articles (the, and, for, in... etc). Just search for the major keywords.
Build on your searches - piece by piece. Pick up keywords from your search results, or authors you may notice reoccurring, or a series of publications that comes up frequently.
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If your keyword is too broad, think about subtopics instead. Example: Instead of searching for "statistics", think about what programs would be included in that category: education statistics, statistics social science, data analysis techniques, etc. Search for those topics instead.
Alternatively, if your keyword is too narrow, think about broader terms. Instead of "measurements in psychology research," you could try "social sciences measurements."
Ask your friendly neighborhood librarian for help! We are happy to assist you in your research. :)