The list of items that meet the criteria specified by the query is typically sorted, or ranked. Ranking items by relevance (from highest to lowest) reduces the time required to find the desired information. Probabilistic search engines rank items based on measures of similarity (between each item and the query, typically on a scale of 1 to 0, 1 being most similar) and sometimes popularity or authority (see Bibliometrics) or use relevance feedback. Boolean search engines typically only return items which match exactly without regard to order, although the term boolean search engine may simply refer to the use of boolean-style syntax (the use of operators AND, OR, NOT, and XOR) in a probabilistic context.