Boolean Logic

Computers can be very effective tools to locate information, but they are ONLY machines. It is important to understand that computer output depends upon human input, and is only as useful as the instructions we provide. Computerized search mechanisms are based on Boolean logic; the better we understand how it works, the better will be the results we obtain.

Boolean logic utilizes three primary operators: AND, OR, and NOT. It is helpful to diagram the effects of these operators:

The first diagram shows two intersecting circles with only their overlapping portions colored. Searching for rock AND roll yields information about a musical genre.

Using the word "AND" actually narrows the results obtained in a search, while using the word "OR" broadens the results.

The second diagram shows two intersecting circles entirely colored. Searching for rock OR roll yields information about igneous rocks, gemstones, crescent rolls, gymnastics, AND music.

To see the difference, enter a custom search for acid rain on Lycos. Try the search once with "OR" and then again with "AND" by checking the appropriate boxes.

Which search gave you the most useful results?

Did you notice? Lycos defaults to search for ANY of the words you enter (Boolean "or"); most search engines assume you want ALL of the words (Boolean "and").

The Boolean operator "OR" can be very useful when there are many synonyms for a concept, and we do not know which one might have been chosen by the author or indexer. For example, teenagers OR adolescents OR youth will yield many more citations than any of these words by itself.

We can also limit the results by using the Boolean operator "NOT".

The third diagram shows three intersecting circles with only the overlapping portions of the top two colored. Searching for rock AND roll NOT Beatles yields information about the Rolling Stones, the Beach Boys, Elton John and others.

Different search engines incorporate Boolean logic in different ways. It is important to read the instructions for each search engine before entering your search terms. See also the section Deciding Where to Search for more detail.

Proceed to Diagramming the Search.

Go back to Constructing the Search.