I hope this isn't too off topic but I'm curious.

I can see how age of domain and P3P can be considered indicators of reliability or trustworthiness of a page and that these would be fairly easy for Google to incorporate into their algorithms but is there any evidence that either accessibility or usability are also factors that they use? If so, how?

I can understand that accessibility is fairly straight forward to ascertain, Google's analysers simply look for tags like; alt, longdesc, summary, label, title and tab index. But what would be the quantifiable indicators of usability? Valid markup doesn't in anyway indicate ease of use or good design. Succinct textual content and clear, logical navigation is easy for users to spot but how would this be added to an algorithm?

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Pleasure in the job puts perfection in the work. Aristotle (384-322 BC)