How Google's Human Search Quality Raters Assign a URL Rating
Google ran over 20,000 experiments in 2010 to come up with the 500 modifications they felt best improved search results for users. While there are over 200 factors, or signals, these constant updates affect their relevance and how they impact search results. It might seem like the algorithm is changing so often, webmasters can’t keep up - especially when updates like Panda come out.
However, there are specific qualities and attributes real people - Google’s URL Raters - look for when they visit your page. Those guidelines have been leaked in the form of a 120+ page training manual for new URL raters.