A. How and when to focus

Say we search Ovid MEDLINE (from 1996 on) for fluoridation:

After Ovid maps our term to the Subject Heading FLUORIDATION, we have the option of checking FOCUS. Let's say we decide NOT to check it:

We get 527 articles:

All 527 articles deal with fluoridation, but some do so in more depth than others. When an indexer assigns Subject Headings to an article, he also marks one or more of those Subject Headings with an asterisk, to indicate that they are the focus of the article.

E.g., for the article "The fight over water fluoridation in Israel", the indexer put an asterisk next to Fluoridation:

*Fluoridation

But for the article "Prevalence and determinants of enamel fluorosis in Flemish schoolchildren", the indexer did not put an asterisk next to Fluoridation. The abstract for this article begins:

"As part of an epidemiological study on the oral health of Flemish schoolchildren, fluoride use was studied together with risk factors (medical history, tap water fluoride concentration, use of fluoride supplements, toothpaste and brushing habits)..."

Fluoridation *Fluorosis, Dental

In some of our 527 articles, there is an asterisk next to Fluoridation, but in some there isn't.

Now let's say we do the same search, but this time we DO check FOCUS:

This time we get 306 articles:

Our results again include "The fight over water fluoridation in Israel", but this time they do NOT include "Prevalence and determinants of enamel fluorosis in Flemish schoolchildren".

Compare the results:

So by focusing, we excluded over 200 articles.

So, should you focus your searches? It's up to you. It depends whether you only want articles that really focus on your topic, or whether you want to find everything on your topic. If you find you're getting too many results, focusing can be a great way of paring them down.