Tim Hunt

Given how Moodle represents the data, it would in theory be possible to compute a facility index for a question worth 0 marks, but based on your findings, I guess it does not do that - and I am not in the mood to dive into the code at this time on a Friday evening to find out.

If you want a work-around (that is probalby only viable if the quiz, or the review, is hidden from students): you could change the quiz to make all the questions worth one mark, then run the statistics repory and download the results; then change the scoring back to how it should be.

I don't think there is any fixed limit like that.

If these questions have qtype='random' then they don't matter, and will probabl get cleaned up at some point.

If so, probably a low risk way to make this problem go away, if you can update things in the database, would be to just change those 8 stamps to any other random string, so they are all different.

Have you turned on Developer Debugging? Also, any of those files giving 'Internal server error', try right-clicking on one and choosing 'Open in new Tab' or similar to see if that reveals any errors.

Does your Moodle site have any additional plugins installed? In particular, any quetsion import/export plugins?