# Difference between revisions of "Using certainty-based marking"

Note: You are currently viewing documentation for Moodle 2.8. Up-to-date documentation for the latest stable version of Moodle may be available here: Using certainty-based marking.

Note: This documentation is based on the more comprehensive Certainty-Based Marking from Tony Gardner-Medwin.

## Certainty-Based marking (CBM)

• After each answer, you say how sure you are that your answer is correct.
• This is on a 3-point scale: C=1 (low), C=2 (mid) or C=3 (high)
• We don't rely on words like 'sure' or 'very sure' because these mean different things to different people
• The mark scheme and the risk of a penalty determine when you should use each C level:

## How CBM works

Certainly level C=1 C=2 C=2
Mark if correct 1 2 3
Mark if wrong 0 -2 -6
Probablility correct < 67% 67–80% >80%
Description Unsure Mid Quite sure
• Certainty levels 1, 2, 3 always give you marks 1, 2, or 3 when you are correct
• If you are wrong, then unless you opted for C=1 you will lose marks: -2 at C=2 and -6 at C=3

## Why use CBM?

• To make students think about how reliable their answer is.
• To encourage students to try to understand the issues, not just react immediately to a question.
• To challenge: if a student won't risk losing marks if wrong then they don't really know the answer.
• If a student is a careful thinker but not very confident. they will gain in confidence.
• It is more fair - a thoughtful and confident correct answer deserves more marks than a lucky hunch.
• Students need to pay attention if they make confident wrong answers: think, reflect, learn!
• Efficient study requires constantly questioning how our ideas arise and how reliable they are.

## How to decide on the best certainty level

• If you're sure, obviously you do best with C=3. But you will lose twice over (-6) if you are actually wrong!
• If unsure, you should avoid any risk of penalty by choosing C=1
• In between, you are best to use C=2: you gain 2 or lose 2 depending on whether you are right.
• The graph below shows how the average mark at each C level depends on the probability that your answer will be right.
• Suppose you think you only have a 50% chance of being right: The highest graph for 50% on the bottom scale is black, for C=1. So you will expect to boost your marks on average most by acknowledging your low certainty (C=1).
• If you think you can justify your answer well, with more than an 80% chance of being correct, then the red graph is highest, for C=3. Opt for this.

• Note that you can't ever expect to gain by misrepresenting your certainty. If you click C=3 (the red line) when you aren't sure, you will expect to do badly - with very likely a negative mark on average. If you understand the topic well, and think your answer is very probably right, then you will lose if you opt for C=1 or C=2 rather than C=3. You do best if you can distinguish which answers are reliable and which uncertain.

## Scores

For more information on scores (relating to the UCL version of CBM but also relevant to Moodle) please see test scores with and without CBM by Tony Gardner-Medwin.