Got a -0.43 on my DA midterm. That rattled my self-confidence in a fundamental way.
Each question is multiple choice with four options. You assign a probability to each option being the correct answer, the total of all of them coming to 1.0. Your probability, p, for the actual correct answer is then adjusted on a natural logarithmic scale: 100/15 * (1+ ln(p) / ln(4)). Thus, if you assign a probability of 1.0, you get full credit. If you assign a probability of 0.0, you get negative infinity. You're thus advised to pick somewhere in between. As you can see from this curve (http://bit.ly/sZJpu0), it's quite possible to get a negative score if you answer too confidently. I answered too confidently, apparently.