The analysis that runs behind the responses a Learner makes is designed to accommodate instances where a Learner appears to have guessed an answer – this might be reflected by a Learner getting an item correct that all prior data suggests they should have got incorrect. A great deal of trialling and analysis has gone into the design of the current response options. The inclusion of a ’don’t know’ option could encourage students not to attempt items, irrespective of whether they find them challenging or not. We need as much information from the student as possible on which to estimate their ability.