It's a matter of Mathematics. Though I do not know if they are using four suits. I don't think so because then they would have the choice of "Even."

The way I see it is if you're at five or lower, chances are that it's higher. If you're at 6-8...that's a bit of a gray zone. But 9+ chances are in your favor that it's low. Huzzah for taking an applied Mathematics class and learning how to count cards in black jack! :D

@ Ken : *sings the Marvin I Love You Song*