Example:
Q: Does Venus exhibit retrograde motion? (1 mark)
A: No. This is because Venus orbits the Sun and not the Earth.
The first part is correct: Venus does not exhibit retrograde motion. But the explanation is incorrect: the reason Venus doesn't exhibit retrograde motion is because it's closer to the Sun than we are. Mars for example also orbits the Sun and not the Earth, but does exhibit retrograde motion.
Do I award 1 mark or 0? On the one hand, for obvious reasons, the grading scheme only covers whether the student said "yes" or "no". Based on that, I should award 1 mark. Further, if the student hadn't written the incorrect explanation, then the answer is perfect, and it feels wrong to penalize the student for going beyond what the question asks for.
On the other hand, the explanation is clearly incorrect and the student should've known the correct explanation (it's part of the curriculum). It also feels wrong to award full marks for semi-incorrect answers. For example, if the student had written something silly such as "This is because Venus is made of Swiss cheese", do I still award 1 mark?!
Ideally, I'd award 0.5 marks, but for various reasons fractional scores aren't permissible.
Answer
You asked for a yes/no answer (which, as you've discovered, has its disadvantages) and got one plus some other stuff. You should grade the yes/no answer and ignore the other stuff. If you like, you could add a note like "You got lucky! This is actually because..."
The whole point of yes/no or MC questions is that you grade only the answer, and assume that type-1 and type-2 errors cancel out or are normalized out. That paradigm doesn't work if you don't uniformly ignore everything other than the answer.
More concretely: other students likely got this question right using the same incorrect reasoning, but didn't write their reasoning down. There is no way to identify these students; so, you need to make sure they get the same score as this student.
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