PageRank

In the 1998 paper that introduced Google, Sergey Brin and Lawrence Page picture a surfer who keeps clicking links and now and then gets bored and starts on a random page ¹. A page's PageRank is the probability that this surfer is visiting it ¹. The damping factor d is the chance of continuing to click rather than jumping, and the authors usually set it to 0.85 ².

Counting backlinks treats every link the same; PageRank instead weighs each link by the rank of the page it comes from, divided by that page's number of links ¹. So a page ranks high when many pages point to it, or when a few high-ranked pages do ². The scores come from a simple iterative algorithm, repeated until they settle; the paper says rank for 26 million pages could be computed in a few hours on one workstation ³. In the explorable, link Home to Fan page and watch one strong link lift it.

The method was patented with Lawrence Page as inventor and Stanford as original assignee; the patent ranks a document from the ranks of documents citing it, plus a constant chance of a random jump ¹. Google says PageRank was one of its core ranking systems at launch, has evolved a lot, and is still part of those core systems ²³. Its public guide to ranking still names links from prominent sites as one sign of quality ⁴. Related: Hacker News ranking, Reddit hot ranking, LinkedIn feed ranking.