Two first-year engineering mathematics courses used computer-aided assessment (CAA) to provide students with opportunities for formative assessment via a series of weekly quizzes. Most students used the assessment until they achieved very high (>90%) quiz scores. Although there is a positive correlation between these quiz marks and the final exam marks, spending time on the CAA component of the course was negatively correlated with final exam performance. Students across the ability spectrum reduced their time commitment to CAA in their second semester, with weaker students achieving lower quiz totals, but with more able students' quiz marks hardly affected. Despite this lower quiz performance, the weaker students still improved their final exam marks in the second semester.