3.2 Boolean Logic

Boolean logic allows you to combine words and phrases into search statements to retrieve documents from searchable databases. So in this session, I’ll show you how to utilize Boolean logic to search the Internet. The Internet is a vast computer database. As such, its contents must be searched according to the rules of computer database searching. Much database searching is based on the principles of Boolean logic. Boolean logic refers to the logical relationship among search terms, and is named for the British-born Irish mathematician George Boole. On Internet search engines, the options for constructing logical relationships among search terms often modify the traditional practice of Boolean searching. Boolean logic consists of three logical operators: Or, and, not Each operator can be visually described by using Venn diagrams. Let’s say I would like information about college. In this search, we will retrieve records in which AT LEAST ONE of the search terms is present. We are searching on the terms college and also university since documents containing either of these words might be relevant. This is illustrated by: the shaded circle with the word college representing all the records that contain the word "college" the shaded circle with the word university representing all the records that contain the word "university" the shaded overlap area representing all the records that contain both "college" and "university" OR logic is most commonly used to search for synonymous terms or concepts. Here is an example of how OR logic works. The search query “college” alone may yeild 396,482 results, the search query “university” may yeild 590,791 results but the search query “college OR university” may yield 819,214 results—much more than if each term was submitted as a stand-alone query. OR logic collates the results to retrieve all the unique records containing one term, the other term, or both of them. The more terms or concepts we combine in a search with OR logic, the more results we will retrieve. And logic works a bit differently. Let’s say I'm interested in the relationship between poverty and crime. In this search, we retrieve records in which BOTH of the search terms are present This is illustrated by the shaded area overlapping the two circles representing all the records that contain both the word "poverty" and the word "crime" Notice how we do not retrieve any records with only "poverty" or only "crime“ Here is an example of how AND logic works. The query “poverty” may yield 76,342 results. The query “crime” may yield 348,252 results. But the search query “poverty and crime” will yield much less than either of those queries if they were submitted as stand-alone queries thanks to the AND operator. The more terms or concepts we combine in a search with AND logic, the fewer results we will retrieve. Finally, we have the NOT logic. Let’s say I want information about cats, but I don't want to see anything about dogs. In this search, we retrieve records in which ONLY ONE of the terms is present, the one we have selected by our search. This is illustrated by the shaded area with the word cats representing all the records containing the word "cats“. No records are retrieved in the area overlapping the two circles where the word "dogs" appears, even if the word "cats" appears there too. NOT logic excludes records from your search results. Be careful when you use NOT: the term you do want may be present in an important way in documents that also contain the word you wish to avoid. For example, consider a Web page that includes the statement that " cats are smarter than dogs." The illustrated in this example would exclude this document from your results. Searchers may also combine operators. For example, let’s say I want information about the behavior of cats. You can combine both AND and OR logic in a single search. The use of parentheses in this search is known as FORCING THE ORDER OF PROCESSING. In this case, we surround the OR words with parentheses so that the search engine will process the two related terms as a unit. The search engine will use AND logic to combine this result with the second concept. Using this method, we are assured that the semantically-related OR terms are kept together as a logical unit. Very algebraic in nature. See. Who says there’s no need for algebra.