SEO by the Sea recently had a post on Yahoo's internal relevancy testing measures and tracked that to a patent filed by them.

An excerpt from SEO By the Sea

How well do the searches for the top related terms in query logs match up with appearances of those top related terms in search results for the primary search term or phrase?

If they match up well, then you might be able to say that the search engine is providing relevant results. If the frequencies of appearances (percentages) don't match up well, then it's possible that a search algorithm or two might need to be tweaked by a search engineer.

The method is used for finding out the relative relevance of results to the primary search term.

It is great to see Yahoo innovate on the search front but can that be converted to a lead over Google?