Certainty-based marking requires you to indicate how reliable you think your answer is. The available levels are:

Certainty level     | C=1 (Unsure) | C=2 (Mid) | C=3 (Quite sure)
------------------- | ------------ | --------- | ----------------
Mark if correct     |   1          |    2      |      3
Mark if wrong       |   0          |   -2      |     -6
Probability correct |  <67%        | 67-80%    |    >80%

Best marks are gained by acknowledging uncertainty. For example, if you think there is more than a 1 in 3 chance of being wrong, you should enter C=1 and avoid the risk of a negative mark.
