Interactive computer-marked assignments (iCMAs) are in use on a range of Science Faculty level 1 modules at the Open University UK, alongside tutor-marked assignments, for summative and formative purposes. They are also used diagnostically to help prospective students to make an appropriate choice of starting point. iCMAs have been very well received by students, but minor problems can easily lead to a loss of confidence. Students engage more with the questions when they carry some weighting and most students feel that their mark for iCMA questions should count towards their overall course score. Two case studies are presented, showing how evaluation has led to further improvements at both the question and whole assignment levels. (Contains 7 figures and 2 tables.)