The aggregation of usability problems is an integral part of a usability evaluation. Numerous problems can be revealed and given that there are usually limited resources for fixing or redesigning the system then prioritisation of the problem set is essential. This paper examines the prioritisation of usability problems from a single heuristic evaluation and multiple heuristic evaluations of Questionmark Perception, a computer assisted assessment application widely used within educational institutions. Two different methods for prioritisation are critiqued; one based on the severity ratings alone and the other on a Damage Index formula proposed by the authors. The results highlight the difference in ranking of problems dependent upon the approach taken. The Damage Index offers a method of systematically prioritising the usability problems in a repeatable way, removing subjectivity from this process, therefore offering improvements over just the reliance upon the severity ratings alone.