There was an interesting presentation about Confidence Based Marking at the Shock of the Old 2005 conference (Oxford 7 Apr 2005). Tony Gardner-Medwin from University College London illustrated how CBM could be used as part of an approach to Computer Assisted Assessment which moves away from testing just surface learning and which enables CAA to test for knowledge, uncertainty, ignorance, misconception and delusion. At risk of a gross simplification of the concept, the principle seems to be declaring a high confidence but selecting the wrong answer costs you big time, so students only tend to do this when they really know, or when they are delusional:) If you want to know more the visit Tony's LAPT site.