deadsea - 10:38 am on Oct 9, 2012 (gmt 0)
I'm convinced that they use bounce-back rate. IE, the user clicks back to the SERPs quickly and clicks another site instead, or refines their search.
We know they have the technology to track this. They can even differentiate between users who open multiple tabs vs users using back button. I've seen them put a notice in the SERPs under sites that I have bounced back from saying to effect "do you want to report/block this site?"
I've heard that this is a "noisy" signal. But I'm sure they use it in the extreme cases where they have enough data and that the rate is well out of the ordinary.
We started collecting data on users interacting with a page in any way. We knew whether they scrolled to the end of the article, clicked to other pages, click on ads, moved a map, played a video, etc. When users didn't do any of these things we assumed they used the back button. We found huge correlation between this metric and the rankings of the pages for their targeted keywords.
We also found huge correlation between the amount of content on the page and bounce back rate. When there was minimal content (just a product name, and a bunch of "be the first to...") the bounce back rate could be 90%. When we had a full complement of content (reviews, prices, places to buy, photos, videos, professional review links) the bounce back rate could be as low as 15%.
We concluded that either Google had a very sophisticated algorithm to measure the amount of content on a page, or that they were doing a very straightforward measure of bounce-back and using that heavily to rank web pages for queries.