UMLUse Case DiagramsFrequency and Criticality

Understanding how often a user will perform a task and how critical that task is to that user is a fundamental aspect of many HCI approaches.

Frequency

Frequency of task (by user) tells the usability practitioner which tasks are frequently performed, which are seldom performed, and which occur regularly though not often. Tasks that are frequently performed should have their user interface optimized for efficiency because any time savings performing these tasks will be cumulative. That is, time savings will accumulate with every sucessful completion of each of these tasks. Tasks that are seldom performed may not benefit as much from optimization. Since the task is performed infrequently, no great time savings will accrue. Conversely, tasks that are seldom performed may require a different kind of optimization. The user may benefit from reminders as to how to perform the task. However, it is a judgment call as to whether or not the user benefits outweight the development resource required.

Criticality

Criticality tells the usability practitioner which tasks are critical to the performance of the user's job and which are of little importance. All critical tasks should be targeted for increased usability effort in design. This is true even of low frequency tasks. Under no circumstances should critical tasks exhibit severe usability problems as this implies that critical tasks are either not completed or take too much time to complete.

Duration

In addition to Frequency and Criticality, one other user by task parameter is of interest: Task Duration. Task duration is a best effort estimate—sometimes merely an order-of-magnitude estimate—of time to complete the task. Tasks that take a particularly long time to complete may, depending on their frequency and criticality, be worth devoting usability resource to. If a low-frequency, medium criticality task takes a long time to complete and it appears that you can substantially cut that time, consider putting some effort into optimizing that task's user interface.