My student: "As of now it is unknown, why...". I have very recently found out why. Do I tell him? I'm supervising an undergrad student. STEM, if that's relevant.
Due to Corona, his contract ending and me leaving this job we haven't spoken in quite some time. He has now worked at home and has written his thesis. I am currently proof-reading his (pretty much) completed thesis which he send to me on 4 days ago.
In his thesis, and obviously last time he spoke, he didn't know why there was a bump in his results. Neither did I. In his thesis he writes something along the lines of &quot;as of now it is unknown...&quot;. So far, so normal.
Two days ago I more or less coincidentally found out why. It turns out it is somewhat obvious once you know why. What should I do now?
I could write him a mail explaining why. But this wouldn't be his work. It also can't be solved. It is just a &quot;quirk&quot; of the method he used.
I could just say nothing: &quot;as of now it's unknown&quot; is perfectly acceptable, but I feel in this case it's also unnecessary. After all he COULD know why.
I thought about not saying anything and just be happy about a question I already have for the oral examn. But I am afraid it might be a little too difficult for the exam and he doesn't do too well with pressure.

Thank you all for your input. You helped shaping my thought process in a great deal.
I ended up writing a detailed email telling him which dots to connect and leaving the rest to figure out for himself with some pointers.
I did so for a simple reason:
His statistical analysis is wrong. Furthermore I don't understand from his thesis what he did. And after all I KNOW what he wanted to do. And I know for a fact: Somewhere something went wrong. And that I can't &quot;let slide&quot;. Incompletion is one thing, errors another.
And in order to provide an explanation I needed to show him why his data doesn't make any sense which is easiest to explain with the underlying mechaism and so on.
I will, however, use this specific topic in the oral exam. I specifically told him to make sure he knows what's happening at this particular point and I feel this is more of a warning than he probably deserves.
This sounds like an easy solution. You can give a couple of suggestions while stressing one as more likely to yield an answer, or even a single suggestion that &quot;may or may not work but could be an improvement on existing work&quot;. This is valid supervisory guidance, as it does not hold the student's hand but provides a direction to be explored and you are not providing a ready answer. The fact that you already know the answer is irrelevant. If the student is unable or unwilling to do so, this is not up to you. In fact, I think is it poorer supervision if you do not point out the weakness but simply let it pass.