Ranking

A Boolean search for an item within a group of items will either return the exact matching item or nothing. This is a rather orthodox search method where the equality between the desired item and the actual item must be exact. In application, it is sometimes far more beneficial and useful to incorporate a more lax measure of similarity between the desired item(s) and the items that exist in the group being searched.

For example, instead of finding only the exact book in a library, a library search engine may return a list of 'similar' books, with the exact book listed first.

The list of items that meet the criteria specified by the query are typically sorted, or ranked, in some regard so as to place the most 'relevant' items first. Placing the most relevant items first reduces the time required by users to determine whether one or more of the resulting items are sufficiently similar to the query. It has become common knowledge through the use of Web search engines that the further down the list of matching items you browse, the less relevant the items become.