A backlink for a given web resource is a link from some other website (the referrer) to that web resource (the referent). A web resource may be (for example) a website, web page, or web directory. A backlink is a reference comparable to a citation. The quantity and sources of backlinks for a web page are among the factors that Google's PageRank algorithm evaluates in order to estimate how important the page is. The PageRank score is, in turn, one of the variables that Google Search uses to determine how high a web page should go in search results. This weighting of backlinks is analogous to citation analysis of books, scholarly papers, and academic journals.