"We are trying to understand what the user is trying to do right now…In some cases, those queries are ambiguous, so you need a little more context...It is probably not just the previous query that matters...You want to know if the user is still doing the same thing. You wouldn't want to go back a month. You wouldn't want to go back a day. But you may want to go back two or three queries."
Posted by Rene LeMerle at 12:21 PM GMT
Unfortunately, the better your Quality Score is the more Google will use Expanded Broad Matches and actually reduce the quality of traffic you receive by opening it up to "synonyms" that no rational person would consider to be the same thing. They are using "user behavior" (click through rate)to determine if these expanded matches are "appropriate to keep serving up, but this appropriateness has nothing to do with relevance.http://www.search-mojo.com/wordpress/?p=200