More sophisticated ranking algorithms

An example of a more sophisticated search ranking algorithm, Google's PageRank rates page quality based upon the quantity and importance of incoming links.[6] PageRank estimates the likelihood that a given page will be reached by a web user who randomly surfs the web, and follows links from one page to another. In effect, this means that some links are stronger than others, as a higher PageRank page is more likely to be reached by the random surfer.

The PageRank algorithm proved very effective, and Google began to be perceived as serving the most relevant search results. On the back of strong word of mouth from programmers, Google became a popular search engine. Off-page factors such as PageRank and hyperlink analysis were considered as well as on-page factors to enable Google to avoid the kind of manipulation seen in search engines focusing primarily upon on-page factors for their rankings.

Although more difficult to game PageRank, webmasters had already developed link building tools and schemes to influence the Inktomi search engine, and these methods proved similarly applicable to gaining PageRank. Many sites focused on exchanging, buying, and selling links, often on a massive scale. Thus an online industry spawned focused on selling links designed to improve PageRank and link popularity.

To reduce the impact of link schemes, search engines have developed a wider range of undisclosed off-site factors they use in their algorithms. As a search engine may use hundreds of factors in ranking the listings on its SERPs; the factors themselves and the weight each carries can change continually, and algorithms can differ widely. The four leading contextual search engines, Google, Yahoo, Microsoft and Ask.com, do not disclose the algorithms they use to rank pages. Some SEOs have carried out controlled experiments to gauge the effects of different approaches to search optimization. Based on these experiments, often shared through online forums and blogs. SEO practioners may also study patents held by various search engines to gain insight into the algorithms.