By 2004, search engines had incorporated a wide range of undisclosed factors in their ranking algorithms to reduce the impact of link manipulation. In June 2007, The New York Times' Saul Hansell stated Google ranks sites using more than 200 different signals.[10] The leading search engines, Google, Bing, and Yahoo, do not disclose the algorithms they use to rank pages. Some SEO practitioners have studied different approaches to search engine optimization, and have shared their personal opinions[11] Patents related to search engines can provide information to better understand search engines.[12]
In 2005, Google began personalizing search results for each user. Depending on their history of previous searches, Google crafted results for logged in users.[13] In 2008, Bruce Clay said that "ranking is dead" because of personalized search. He opined that it would become meaningless to discuss how a website ranked, because its rank would potentially be different for each user and each search.[14]
In 2007, Google announced a campaign against paid links that transfer PageRank.[15] On June 15, 2009, Google disclosed that they had taken measures to mitigate the effects of PageRank sculpting by use of the nofollow attribute on links. Matt Cutts, a well-known software engineer at Google, announced that Google Bot would no longer treat nofollowed links in the same way, in order to prevent SEO service providers from using nofollow for PageRank sculpting.[16] As a result of this change the usage of nofollow leads to evaporation of pagerank. In order to avoid the above, SEO engineers developed alternative techniques that replace nofollowed tags with obfuscated Javascript and thus permit PageRank sculpting. Additionally several solutions have been suggested that include the usage of iframes, Flash and Javascript.[17]
In December 2009, Google announced it would be using the web search history of all its users in order to populate search results.[18]
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