Combine your searches with OR when you want to create one big group on a single concept. Use OR when you combine synonyms, alternative spelling, text words and subject headings on the same concept. OR gives you mORe results. In the diagram below, ALL blue areas would be included in your results, minus any articles that had appeared in more than one set.
Combine with AND when you want to combine two or more distinct concepts. Combining with AND will narrow your search results, and the resultant set will contain articles that have all the concepts you have combined together. In the diagram below, the intersecting section between the blue and green sets will be what you retrieve. Articles on both aged AND falls will be in your results.
See below for a working example of how a search can be structured using OR and AND to retrieve a more relevant set of results. You'll notice that each of the concepts from the search topic (risk factors of falls in elderly hospital inpatients) have been dealt with in the same way; subject heading search(es), textword searches, then combined with OR. Once all concepts have been explored, the big sets that have been put together with OR are then combined with AND.
Below are a couple of final observations on the search process.