Tim Hunt

The field mdl_quiz.previous_link should not exist. At least, it has never existed in a Moodle release. Someone may gave added it to your Moodle as a customisation. If so, you will have to sort that out yourself.

Do you want students to be able to review their previous attempts at all? If not, you can just turn off those review options in the quiz settings.

If you set the group to Separate of Visible groups mode in the quiz settings, then you can run the statistics report for each separate group. That will probably do what you want. To get side-by-side comparison, you will probably need to download the separate report for each group, and then combine them yourself.

It would be possible to make a new quiz report plugin that automated that. (It could call the same functions that the statistics report uses.) Wee http://docs.moodle.org/dev/Quiz_reports for details. Whether doing that is hard or not depends on our development skills. I suggest you try to understand the code in mod/quiz/report/statistics. If you can understand that, making the combined report is probably easy. Otherwise it is probably hard.

The list of columns you give is the list of columns in the question_attempt_step_data table.

Well, any sort of test of different users simultaneously attempting a quiz, and compare performance of 2.3.x and 2.4 beta.

Right now, the details probably are not important. Any sort of test like this is better than nothing. Once we have some data, we may then want to refine things, but for now there is a vacuum to fill.