Scaling Usability Based on Requirements

When a company such as a bank hires a usability engineering consultant, they usually have a specific problem they are trying to solve. One common problem is that customer-service call centers are too expensive to operate. When it is not practical to build a self-service website for their customers, clients will choose instead to redesign their call center agent workstations to make them more efficient. Sometimes, just reducing the amount of time by a few seconds an agent needs to complete a customer service interaction can save the company hundreds of thousands of dollars. The company may even calculate how much time they need to save on certain high-volume customer service interactions in order to achieve their business goals. That targeted time savings then becomes what we call a usability requirement. Of course a usability requirement can also be defined in terms of the number of errors an agent makes using their workstation to complete an interaction, or even just the number of clicks.

Regardless of what the formal usability requirement may be, the usability engineer needs a way to estimate the likelihood that a proposed user interface design will meet that requirement. In other words, is there a way to scale measured performance in a usability test against some criterion so that it represents the probability of achieving a specific usability requirement?

Abstract

Software usability should be scaled in terms of the probability that a user interface will meet established usability requirements. This paper describes a scaling procedure that first estimates the distribution of common usability metrics and then calculates the probability that a user interface will meet a specific requirement. A series of Monte Carlo simulations showed that even with a small sample of usability test participants, the scaling procedure remained unbiased and could accurately differentiate usable from unusable user interfaces.

Citation

Fracker, M. L. (2010). Scaling usability in terms of requirements: A method for evaluating user interfaces. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 54(6), 576-580.

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