I'm sure someone/somebody/organization has stats on this...probably not specifics but I've been privy to some discussions between health professionals while discussing THC test results and they definately knew who tested "high" or "low" on a general scale.
But like I said, you could theoretically be high the day tested, and have a negative result, or possibly a low THC metabolite count if you hadn't smoked for awhile.
Yet someone who smoked the day before the test, but is sober during work, could have a higher count on the test.
It's a good test if you are looking for usage at any point in time, but not if you are screening between people who only smoke at home versus the people who smoke and are high at work.
When it is, at some point, likely to become a substance that is used like alcohol, where you enjoy a good toke after work (not necessarily to the point of getting stupid-stoned)... there has to be a way to screen out the users who stay completely clean during work hours. Some businesses probably wouldn't care if it was legal and you kept it at home.