Oh, you thought I had the correct answer?
Sorry. I don't.
But The Mathlete over at MGoBlog has done some deeper digging than you or I would ever care to do:
http://mgoblog.com/diaries/rating-raters-reviewing-recruiting-services-success-and-bias
Kinda long, but a decent evaluation.
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"I don't get sick. I don't observe major holidays. I'm a jackhammer." - Jim Harbaugh & Dwight Schrute