Distractors for multiple-choice questions

I’ve just been asked a question (well, actually three questions) about the summative use of multiple-choice questions. I don’t know the answer.┬áCan anyone help?

If we want 3 correct answers, what’s the recommended number of distractors?

If we want 4 correct answers, what’s the recommended number of distractors?

If we want 5 correct answers, what’s the recommended number of distractors?

I have [31st October] now found out a little more and the answer has to be completely correct and no partial credit is allowed. I have also realised that the answer lies in our own previous work on random guess scores. We’ve repeated the sums for these particular examples (see multiple-choice-distractors) and my recommendation to the module team is that if they require 8 options for each question (so, if they require 3 correct answers there are 5 distractors; if they require 4 correct answers there are 4 distractors etc.). The probability of getting the question right completely by chance will then always be less than 2%, and the probability of getting multiple questions right in this way is vanishingly small.

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