The link between absenteeism and students' academic performance at university is perpetually a hot topic for teaching academics. Most studies suggest the effect is negative, although the strength of this effect is in dispute. The issue is complicated further when researchers draw their inferences from different angles, such as the removal of a mandatory attendance policy or the implementation of a module-specific attendance policy. Although previous studies have suggested the effect on exam performance of removing a mandatory attendance policy is weak, this study investigates the effect of implementing a module-specific attendance policy and finds a strong effect on exam performance. We also identify that student-specific factors are important, including revision strategies and peer-group effects and that not taking account of these factors will result in biased estimates of the effect of an attendance policy on exam performance. Furthermore, this paper suggests that the effect of absenteeism on exam performance is non-linear and further research is needed to identify when attendance policy is a justifiable tool. (Contains 1 figure, 8 tables, and 8 notes.)