The second aspect of relevance that we consider is whether it is binary or multivalued. Binary relevance simply means that a The Boolean retrieval model was used by the earliest search engines and is still in use today. It is also known as exact-match retrieval since documents are retrieved if they exactly match the query specification, and otherwise are not retrieved. Although this defines a very simple form of ranking, Boolean retrieval is not generally described as a ranking algorithm. This is because the Boolean retrieval model assumes that all documents in the retrieved set are equivalent in terms of relevance, in addition to the assumption that relevance is binary. The name Boolean comes from the fact that there only two possible outcomes for query evaluation
(TRUE and FALSE) and because the query is usually specified using operators from
Boolean logic (AND, OR, NOT). As mentioned in Chapter 6, proximity operators and wildcard characters are also commonly used in Boolean queries. Searching with a regular expression utility such as grep is another example of exact-match retrieval.
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