TrustRank
Preliminary source: https://dailymemphian.com/article/242/Stalled-out-in-search-ranking-Consider-your-PageRank-and-TrustRank
When Google made public their use of PageRank in their search results, it unfortunately led to a flurry of people trying to game the system and artificially boost their PageRanking. As a PageRank is based on how many other websites have links to the page, spam websites popped up that people could pay to access to boost their webpage’s PageRank. As the clout of the webpage that contained the backlink mattered in calculating PageRank, those spam webpages had to have high PageRanks of their own, so they likely referred to each other.
A method for countering these pages created to mislead search engines was developed by researchers of Stanford and Yahoo!. This method, called TrustRank, was an algorithm that would follow the idea that “birds of a feather flock together.” The method starts with a manual analysis of a small set of webpages which are classified as either spam or real. The assumption was that in general, pages that were real and had PageRanks based on organic growth and popularity would be referred by other real pages, and similarly, the spam pages would be referred/backlinked by other spam pages. Using this idea, each page would be assigned a TrustRank based on the ratio of real sites it is referred by. The accuracy/success rate of TrustRank isn’t guaranteed though. Similar to the concept of photocopying photocopies rather than the original, the farther removed the algorithm crawls from the initial set of webpages, the less accurate it is.