Basic documentation on Certainty-Based Marking (CBM) is at https://docs.moodle.org/en/Using_certainty-based_marking.

This plugin is required to display Staff Reports that properly summarise student performance in quiz attempts using CBM behaviour. It is essential if staff are to see clear reports for quizzes that use CBM. The plugin should really be in core Moodle, since CBM is an option in core Moodle, which enables students to see full reviews when CBM is in use, even without the plugin. 

The plugin allows staff to opt whether or not to see student scores on individual questions. Percentage scores are calculated on the basis of either just those questions for which a student has entered a response, or the whole quiz. For example, 100% correct on half of the questions - those which the student has chosen to work on - can be shown as 100% rather than 50% correct. This is appropriate when a student uses the quiz for self-testing, choosing just questions relevant to current learning or self-assessment needs. 

Core Moodle (and this plugin) use the average CBM mark (max=3) to generate Grades up to 300% if all answers are both correct and given with high confidence.  An alternative grading scheme is available with "CBM plus" code downloadable from https://tmedwin.net/cbm . This gives grades that are more directly comparable with conventional accuracy.  CB  Accuracy  =  Accuracy + a  CB Bonus  (typically 0-5%) that rewards students who correctly distinguish sound and unreliable answers. The issues are discussed both in Moodle at Using certainty-based marking,  and at the download site.

Please report any issues to me: Tony Gardner-Medwin (a.gardner-medwin@ucl.ac.uk)