# What is Certainty-Based marking (CBM)?

• 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:
Certainty levelC=1C=2C=3No Response
or 'No Idea'
Mark if correct123( 0 )
Mark if wrong0-2-6( 0 )
Probability correct< 67%67–80%>80%-
DescriptionUnsureMidQuite 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 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 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. You might be lucky; but on average you will lose! If you understand the topic well, and think your answer is reliable, then you will lose if you opt for C=1 or C=2 rather than C=3. You will do best if you can distinguish which answers are reliable and which uncertain.