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The first step in choosing keywords is to think about what information you need to search for. This seems obvious, and to a certain extent you are probably already doing it. But a more systematic approach will reward you.
1. Break the search down into the key concepts/ideas. For example, if we are looking for information about nutrition and cancer the main concepts are nutrition AND cancer.
2. Brainstorm (or look for in your background reading) synonyms since not all authors or websites will use the same words or terminology. Making a concept box can greatly aid you in your search.
or specific type of cancer (e.g. breast cancer)