Certainty-based marking is all about the student being honest about what they do and don't know. Therefore, it is contradictory to encourage guessing, and we should provide an explicit 'I don't know' choice along side the three certainty options.
When I don't know is selected, any response given is not graded, and the question ends up in the 'Not answered' state.
Some questions about how to implement this:
1. Should 'I don't know' be treated as 0 marks or the random guess score? (Using the random guess score is mathematically 'more right', in that it removes the incentive to guess, but it may be psychologically wrong.
2. If the student happens to put in a response to the question, as well as select 'I don't know', should we show them whether their choice was correct or not, and the feedback, or should it really be as if they did not input a response?
3. If the student forgets to select a certainty choice (or I don't know) which option should we assume? I think we should continue to treat that as low certainty.