" Oh, I think so. Yeah. Oh, well, sure. A ‘type-token’ ratio, I suppose, would be as good a way as any to work that out, and with samples of a thousand words or more, you could just check the frequency of occurrence of the various parts of speech.” “And would you call that conclusive?” “Pretty much. You see, that sort of test would discount any change in the basic vocabulary. It’s not the words but the expression of the words; the style. We call it ‘index of diversity.’ Very baffling to the layman, which, of course, is what we want.” The director smiled wryly. Then he nodded at the tape in Karras’s hands. “And so this other person’s voice is on that one?” “Not exactly.” “Not exactly? "

― William Peter Blatty , The Exorcist