You must balance being as comprehensive as possible, while staying to topic.
Example (right): In PsycINFO, it would be more accurate searching to limit to a human Population Group than to include the word human in your search terms.
If you did not have the option to use a limiter, you would want to search in a Methodology or Abstract field, where the population group is most likely to be discussed.
AND, OR & NOT are Boolean search operators. You can use these words to direct a database how to search for your concept terms.
You can combine AND, OR, and NOT to build very complex searches by grouping each concept in parentheses.
(surcalose OR aspartame OR saccharin) AND headaches
You can use OR with dissimilar concepts, but watch out for the relevancy and result size. This example would need refining, but could be a useful starting point.
(cats OR dogs) AND "household pets"
In the following example, I want to look at different types of therapy that cats and dogs are used for, but I'm not interested in therapy to remove phobias about dogs or cats. Also, I happen to know that authors Samuel and Elizabeth Corson do research on drug therapy used for aggressive animals. That would be the wrong direction, so I want to exclude their articles.
Set searching allows you to break apart sections of a Boolean search and recombine them. Each search you try becomes a line in a grid. At any time you can check off a box to combine lines by a Boolean command.