Your roommate is out. Your favorite porn site was just updated, and you've got one thing on your mind. Is it safe to whip it out and start going at it, or will you be caught with your hands covered in shame?
0 7 points: Put it away. It's not worth the risk. Your roommate may be gone, but he could walk in at any moment. He may even come back with a big group of friends. There's no way to hide it if you're caught. Sitting hunched over your computer with a pillow on your lap is the international sign for "I was masturbating" and it won't fool anyone. You're better off waiting for a safer time, like 20 minutes after he left for class. Better yet, wait for him to go home for a long weekend so you can beat off at your leisure without having to worry at all.
8 13 points: It's dangerous. The results are too inconclusive to give you a definite answer either way. You might be able to get away with it if you're fast. Try to avoid it unless you're really, really desperate.
14 21 points: Buy yourself a one-way ticket to orgasm-town, you're in the clear. The stars are aligned in your favor. The conditions are perfect. There is very little chance your roommate will come back for at least another 20 minutes. Remain vigilant, though. There are some factors which can never be predicted, such as canceled classes, forgotten books and tests that end early. Always be ready to put your thingy away at a moments notice.