It's partially just a numbers game. If you assume the top third of candidates only need 2 interviews to get an offer, the middle third need 5, and the bottom t

It's partially just a numbers game. If you assume the top third of candidates only need 2 interviews to get an offer, the middle third need 5, and the bottom third need 15, then 15 / (2 + 5 + 15) = 68% percent of the candidates you interview will be from the bottom third. This, of course, ignores every human factor but it is a starting point.