Google's rise to success was largely due to a patented algorithm called PageRank that helps rank web pages that match a given search string. When Google was a Stanford research project, it was nicknamed BackRub because the technology checks backlinks to determine a site's importance. Previous keyword-based methods of ranking search results, used by many search engines that were once more popular than Google, would rank pages by how often the search terms occurred in the page, or how strongly associated the search terms were within each resulting page. The PageRank algorithm instead analyzes human-generated links assuming that web pages linked from many important pages are themselves likely to be important. The algorithm computes a recursive score for pages, based on the weighted sum of the PageRanks of the pages linking to them. PageRank is thought to correlate well with human concepts of importance. In addition to PageRank, Google, over the years, has added many other secret criteria for determining the ranking of pages on result lists, reported to be over 250 different indicators, the specifics of which are kept secret to keep spammers at bay and help Google maintain an edge over its competitors globally.