Matt Cutts (from Google) has an enlightening post on his blog titled – What’s an update? which quite simply lays out information on the topic of their updating system. I’m sure it’s more complicated than he’s outlined here but the information resonates with my experience of Google’s updating system as a publisher.
‘What is an update? Google updates its index data, including backlinks and PageRank, continually and continuously. We only export new backlinks, PageRank, or directory data every three months or so though. (We started doing that last year when too many SEOs were suffering from “B.O.”, short for backlink obsession.) When new backlinks/PageRank appear, we’ve already factored that into our rankings quite a while ago. So new backlinks/PageRank are fun to see, but it’s not an update; it’s just already-factored-in data being exported visibly for the first time in a while.’
This is why when I post here that there are changes in backlinks and page rank I often get emails and comments from readers saying that they noticed the changes in these numbers – but that it had no impact upon their actual traffic levels or SERPS (how high you rank for any given search on Google).
In a sense the backlink and page rank updates are only really useful because they give an indication of how a site has been traveling.
What we really should be interested is when Google updates it algorithm – or how they value and score pages. These are the updates that have impact upon the actual SERPS. These changes happen less often than backlink and page rank updates and can have quite significant impact upon a site. I still shudder when I think of the algorithm change that happened mid December last year when my traffic levels dropped to a third of what they were – ouch. Luckily it only lasted 6 or so weeks when things returned to normal.