How do PageRank updates work?

Matt's answer:

Matt Cutts: Today’s question comes from Made, in London who wants to know: “On what basis are you increasing the PR for each page rank update?” It’s a good question, and let’s talk about page rank updates in general. So, just as a review, page rank is Google’s opinion about how reputable a page is. It’s something that is typically computed continuously. We have a bank of machines that’s always looking at new links that we discovered are in the crawl continuously updating. So we have very fine-grained notions of page rank within Google. Outside of Google, page rank is truncated to ten levels that are visible in the Google toolbar. And we typically say, take our opinion about the page rank or the reputation of each page. And every sort of few months, typically it’s about four times a year-so about every three months, we more or less have a chron job. So like April 1st, you know, three months later, you might see a page rank update, and we’ll just go ahead and push that live. We have a few safety checks in place, so we say like “OH! Did we expect the page rank of MSN.com to be 7 or 8 and then suddenly dropped to 2…” If those sorts of things happen, then we might get an email and check into it. But for the most part the process runs with very little human intervention. People don’t really need to oversee things. So, we do still reserve the right, because it is our opinion of the reputation of a page that…For example, if a page is selling links that pass page rank, then we might lower the toolbar page rank. But, for the most part, a lot of this stuff can now run automatically, and the updates pretty much happen like clockwork. So, again page rank is our opinion of the reputation of the page. It includes not only the links to your page, but how important those links are. And then we do that by having a massive amount of computation that sort of says, across the entire web, try to figure out which pages are the most reputable, and then that typically comes out every 3 or 4 months. So that’s kind of a, a rough idea. It is still our opinion, but we also do a lot of the work by machinery. And that machinery on the, on the whole tends to work very well and things just flow out naturally from there.

by Matt Cutts - Google's Head of Search Quality Team

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