Basic documentation on Certainty-Based Marking (CBM) is at https://moodle.org/plugins/view.php?plugin=quiz_cbmgrades .
This plugin displays result reports (for teachers) that properly summarise student performance in quiz attempts that use CBM behaviour. Optionally, scores on individual questions can be displayed. Percentages can be 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.
The plugin was developed alongside improvements in CBM behaviour that are included in core code in Moodle 2.6+. Version 1.2 can be used with Moodle 2.3-2.6. Version 1.4 is required for Moodle 2.7.
The plugin is not what is required to ensure proper feedback of information to students. Proper feedback was introduced in core Moodle in 2.6. Earlier versions require patches to the core code that are available (for Moodle 1.9+) at http://TMedwin.net/cbm/moodle.
Please report any issues to me: Tony Gardner-Medwin (firstname.lastname@example.org)