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According to Dan Sullivan's periodic table, they do measure that. And it has a weight of "2" according to his model. See the "Ce" tag:

http://searchengineland.com/seotable

They do say they check if the same user by cookie or logged in is back on Google within a short amount of time. This would mean the site was a poor fit for the query. It is an indicator of the quality of the result for this query and for this user. They never mentioned using Google Analytics, maybe they do too.

With the amount of money at stake with even a 0.1% improvement to search quality, I must assume that every possible correlation against data they can gather is investigated.