A sample of 202 students filling in a student evaluation of teaching (SET) questionnaire were asked to complete another questionnaire asking about the specific reasons for awarding a score to the specific SET questionnaire items. The aim was to find out what influenced students' judgements on those items. It was found that students' interpretation of some questions differed from the 'expected' interpretation. Several factors, such as the placing of questions and the salience of items retrieved from memory, could influence a score. It was also found that asking for an explanation improved scores overall. The conclusions were that questionnaire completion could be understood as a form of problem solving and judgement under uncertainty. The specific heuristics used led to variability in students' interpretation of the task.