Focus that subject heading

To explain the use of Focus it's necessary to go back to those busy subject indexers who are reading and analyzing journal articles in order to assign the most specific descriptors available to describe the article's subject content.

The subject indexers assign between six and fifteen descriptors; at the end of this process, they select up to four that signify the major points of that article. When you look at the MeSH field in a MEDLINE record, any descriptor marked with an asterisk indicates a major point. Focusing restricts a subject search to only those articles where that subject is one of the major points.

Here are two scenarios where you can use Focus to your advantage:

1. You're looking at diabetes mellitus and coronary artery disease. Beyond that you're not sure how you want to limit the search or what other concepts you might want to add - you're just sort of looking at what's out there. In this case, it's best to focus both diabetes mellitus and coronary artery disease and even then you'll probably have a rather large results set. But in the results set that you do get, you can be certain that your two topics will be the main thrust of the citations. Then, of course, you'd add your other limits (reviews, English, human, year range, etc.). This is a good way of scanning citations to get an idea of what's out there on your topic.
2. You're doing a search on diabetes mellitus and articles about hip replacement appear in the results set. After looking at a few citations you realize that the main point of the these citations is hip replacement and diabetes seems to be a side issue. By Focusing diabetes mellitus, all of the citations where diabetes is NOT one of the main points will drop out. It's a way of choosing which aspect of your search question has a higher priority and the results set will reflect that. Also, you can always 'unfocus' if that strategy doesn't seem to work for your question.

FYI
You can explode and focus at the same time.