I've only taken one Clep so far, and I had no prior knowledge on the subject at all. I used the instantcert flashcards and passes all three Petersons practice exams, and then the Clep.
Best of luck!

Alina

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CLEP
3/07 Sociology 69
4/07 Humaities 65
5/07 Biology 67
5/07 Psycology 67