Why is this necessary? Consider the numbers:
Between February 10th at 7:51 pm and February 18th at 11:43 pm, I received 1,707 e-mails. 1,079 of those were spam. 559 were caught by SpamAssassin; 520 weren't caught.
I get a lot of spam.
If you're too lazy to do the math, I had 628 "ham" (non-spam) messages in that time period. And no, I don't count any old mail as spam--things like old lists don't count.[/edit]