Informing my supervisor of discrepancy in the results I am working on my MPhys thesis, on a certain dataset. Part of this dataset has been analyzed by my current supervisor during his PhD, with published results. He asked me to expand on that work by completing the analysis on the whole dataset.

When I repeat the analysis on the part he had already worked on, I get different results. Unfortunately he has not published any detail about how exactly he tackled the problem, and instead relied on his memory. He gave me advice like: put this parameter to 50, use feature A, use feature B, disable feature C and so on.

I was able to reproduce his result to within 10%, but yesterday I was horrified when I discovered that this was done without using one of the features he recommended, let's say feature A. I have played with the data since and am totally unable to reproduce the result (it is off by a factor of 2 when using my supervisor specifications).

Using feature A totally makes sense, so I am not sure on what to do: I am afraid that should I tell him about this problem, he would delay my upcoming viva. This has a lot of unwanted consequences, for example would force me to pay extra money to the University
In my opinion, it is absolutely essential that you are completely open about this.

If you cannot reproduce his results, at least one (possibly both) of the following are true:


You are doing something wrong. You are a student and he is your supervisor. It's his job to help you if you are doing something wrong. There is nothing to be ashamed of. It's better to be delayed with correct results, than on time with wrong results.
He was doing something wrong. Don't be afraid! You are not accusing him of anything unethical (*). If his previous results or his understanding are genuinely in error, he should be very interested in knowing this, and grateful if you discover such an error.


(*) If you do suspect unethical behaviour, the question is entirely different and better asked separately.