Candidates are encouraged to write full proofs to the questions they attempt, as a full answer to a question is worth many more marks than incomplete answers to several questions. This is because of the marking scheme: an answer is marked on either a "0+" or a "10-" mark scheme, depending on whether the answer looks generally complete or not. So if an answer is judged incomplete or unfinished, it is awarded a few marks for progress and relevant observations, whereas if it is presented as complete and correct, marks are deducted for faults, poor reasoning, or unproven assumptions. As a result, it is quite uncommon for an answer to score a middling mark (i.e. 4–6).