Hello Christoph,
I don't think there is such a plugin, but I could be wrong. I also think that modifying the multiple choice question to do what you want is probably not that easy. I therefore suggest the following:
Tell students what the number of correct choices is and that if they select more answers than that number, a deduction will be applied for the incorrect answers.
For example, for three correct choices, the mark for the correct choices should be 0.33 and that for the incorrect choices should be -0.33 (i.e. negative). That way, the score for the questions will always be either 0, 0.33, 0.67 or 1, regardless of the total number of choices offered and the number of answers selected by the student.
For two correct choices, the mark for the correct choices should be 0.5 and that for the incorrect choices should be -0.5, and so on.
I don't think there is such a plugin, but I could be wrong. I also think that modifying the multiple choice question to do what you want is probably not that easy. I therefore suggest the following:
Tell students what the number of correct choices is and that if they select more answers than that number, a deduction will be applied for the incorrect answers.
For example, for three correct choices, the mark for the correct choices should be 0.33 and that for the incorrect choices should be -0.33 (i.e. negative). That way, the score for the questions will always be either 0, 0.33, 0.67 or 1, regardless of the total number of choices offered and the number of answers selected by the student.
For two correct choices, the mark for the correct choices should be 0.5 and that for the incorrect choices should be -0.5, and so on.