The purpose of this page is to explain the rationale behind the marking of multiple choice tests, which sometimes form part of the assessment for 85349 Operating Systems.

Unlike other tests, however, a multiple choice test already contains the correct answer to every question as one of the possible responses. Pure blind guesswork will therefore result in some correct answers, even for the person who knows nothing at all about the subject.

To correct this anomaly, negative marks need to be introduced for incorrect answers. Any multiple choice test which does not incorporate negative marks as an integral component of the marking scheme will produce invalid results.

For a test which has N possible responses to each question, a negative score of 1/(N-1) should be introduced. So if each question has four possible responses, the penalty for an incorrect answer should be one-third of a mark.

For example: for a test with 20 questions, and 4 possible responses for each question, random guessing should produce 5 correct answers, and 15 incorrect answers, for a score of 5 - 15*(1/3), which equals zero, as required.

Note that where the possible correct response can be narrowed down, it is still very much worth guessing. For example, if the answers to 10 questions are known, and the other 10 can be narrowed down to one of only two possibilities, then leaving these doubtful 10 blank will result in a score of 10/20; whereas if guesses are made, with a one in two chance of being correct in each case, one can expect a score of 10 + 5 - 5*(1/3), or 13.33/20.