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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. Like, 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'll award 0.5 marks, but for various reasons fractional scores aren't permissible.

I think that if you would allow full marks for just yes/no without an explanation at all, then you should allow it here. Otherwise the question is flawed and can't be properly and fairly graded. But a note to the student would be good, also.

To be more precise, if it is possible to answer a question with inconsistent parts it isn't a valid question for examination. It should be clear and clean.

But your job is to educate, not to grade. Give the marks and write the note. And think harder about the questions you ask and how they are presented.

If the explanation is required, it is a different situation. In that case, and if you weight the explanation heavily for other students, then probably 0 marks is better than any other alternative.

Give him the point. If you want, mark on the paper "wrong reason" in red.

If you wanted to evaluate reasons, you would have made it more points and required an explanation. But you didn't. So treat it like a normal true false or multiple choice problem. Reason not graded, just getting the right answer. Luck allowed. Etc. Similarly right reason but wrong result gets hammered.

If providing an answer was required then I guess you could mark wrong any case where both answer and reason were right.

P.s. This is if you are the teacher. If you are the student, don't debate 1 point. Get it all perfect next time.

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.

There are a few schools of thought here, and it really depends on your teaching style.

From a fairness perspective, you shouldn't mark this student down. I'm quite sure that there are a few other students in your class who could not explain why they got the right answer (a good number of them probably just guessed at random). Unless you have a system to find out who those students were and penalize them, I think you'll have a hard time justifying why this student gets marked down and all those other students are not.

From an instructional perspective, there is some merit to marking this student down. Students have a tendency to write down random stuff with the hope that something sticks. If you incentivize your students to write less bulls**t and more to the point, you are teaching them a valuable lesson. You will be signalling to this student that you care about how they reason about answers, and not just the final product. I know of some lecturers who give their students 1 point in essay questions if they write nothing, just to provide an incentive to not write nonsense.

I would lean towards a fair verdict, but this is really because your question was very limited in answer scope.

This is a discretionary matter, and different lecturers will treat it differently, depending on their own educational preferences. However, I disagree strongly with some other commentators on this thread. In my view, there is nothing unfair in marking a student down for unsolicited and incorrect information. Indeed, I would say that this is generally a good practice, since it ensures that the student is held responsible for the correctness of their assertions, even in cases where they offer unsolicited information. This implicitly gives the student some broader training in the importance of ensuring that they give correct information even when they choose to advance information that is unsolicited --- something that is a broader life-skill of importance.

In my personal practice, if a student gives me more information than was requested, and that additional information is wrong, this incurs a marking penalty just as if that information was part of the question. I warn my students in advance that this is my practice, but it is a justifiable practice even without giving a warning. In this particular case, if I were marking the question, I would not give the student full marks.

What kind of graduates do we want? We are training students to become professionals in difficult fields. So, in considering this issue, I think it is important to consider the implicit lessons we give students by what we penalise and what we don't. Imagine that this student graduates and practices in your field. Would it be okay if this practitioner gives unsolicited information to people on the subject area, and that information is wrong? Would you be comfortable working with a colleague who gives information to you or others that is sloppy and incorrect, but then he faces no penalty just because that information was not requested by others? Is that the lesson you would like to impart to your students? Is that what you want to teach them about the world?