Tim Hunt

There should be an explanation at https://docs.moodle.org/dev/Quiz_statistics_calculations, but currently equation rendering is broken there, which makes the page not very helpful. (MDLSITE-8046).

The code that computes the statistics is spread over various classes, but if you want to look, https://github.com/moodle/moodle/blob/3e73c33f9920fbb066f2c554fb998af46d4706d8/question/classes/statistics/questions/calculator.php#L37 is probably a reasonable place to start. (And, how that is called from mod/quiz/report/statistics code.)

I don't think moving questions in the question bank, or editing the default mark of teh questoin in the question bank, would affect existing quiz attempts. So, I don't think this explains it.

I think you are seeing an after-effect of MDL-82772, which has now been fixed. At any rate, the error is arising becuase the random questions are messed up and pointing to a category that no longer exists. The fact that displaying this outputs erros is really a sympton, but the but is how did it get like that?

If you can still reproduce this, please let us know the exact steps so we can fix it.

This definitely should not be happening. The values in the questionid column of the question_versions table should be unique. (Why doesn't the DB schema enforce this? I created MDL-85197.)

In the cases where you have several rows in question_versions with the questionid, which questions are those? can deduce how it got like that?

I have not thought about this in detail, but it is important that the version of goemaxima you use matches the version of STACK that you are using, and this is the kind of error you might get if there is a mismatch.