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Using certainty-based marking

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Revision as of 10:41, 11 November 2013 by Mary Cooch (talk | contribs) (adding Tony M-G's CBM notes to the docs)

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

cfmarks.gif

  • 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.

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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 less than an 80% chance of being correct, then the red graph is highest, for C=3. Opt for this.

cfgraphs3.gif