Page and Brin founded Google in 1998. Google attracted a loyal following among the growing number of Internet users, who liked its simple design. Off-page factors (such as PageRank and hyperlink analysis) were considered as well as on-page factors (such as keyword frequency, meta tags, headings, links and site structure) to enable Google to avoid the kind of manipulation seen in search engines that only considered on-page factors for their rankings. Although PageRank was more difficult to game, webmasters had already developed link building tools and schemes to influence the Inktomi search engine, and these methods proved similarly applicable to gaming PageRank. Many sites focused on exchanging, buying, and selling links, often on a massive scale. Some of these schemes, or link farms, involved the creation of thousands of sites for the sole purpose of link spamming. In recent years major search engines have begun to rely more heavily on off-web factors such as the age, sex, location, and search history of people conducting searches in order to further refine results. By 2004, search engines had incorporated a wide range of undisclosed factors in their ranking algorithms to reduce the impact of link manipulation. Google says it ranks sites using more than 200 different signals.The three leading search engines, Google, Yahoo and Microsoft's Bing, do not disclose the algorithms they use to rank pages. Notable SEOs, such as Rand Fishkin, Barry Schwartz, Aaron Wall and Jill Whalen, have studied different approaches to search engine optimization, and have published their opinions in online forums and blogs.SEO practitioners may also study patents held by various search engines to gain insight into the algorithms.In 2007 Google announced a campaign against paid links that transfer PageRank. In 2009 Google disclosed that they had taken measures to mitigate the effects of PageRank sculpting by use of the nofollow attribute on links.
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