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How Low Can You Go? Is the System Usability Scale Range Restricted?

Philip Kortum and Claudia Ziegler Acemyan

Journal of Usability Studies, Volume 9, Issue 1, November 2013, pp. 14 - 24

Abstract

Previous research suggests that the System Usability Scale (SUS) might not be generating usability scores that span the entire measurement range from 0 to 100, particularly at the lower end of the scale. Most published literature has reported study mean scores that are typically above 40. The use of only a subset of the SUS could change how collected data is interpreted, especially when comparing the relative usability of systems. In this study, participants reported their subjective usability assessments of 14 different voting interfaces using the SUS. Participants were given a packet that contained the 14 ballots presented in random order. After completing each ballot, the participants were given the SUS and asked to rate the usability of the ballot that they had just used. Results showed that nearly the entire range of the available scale was used, with average study scores ranging from 15.4 to 93.0. Nine of the 14 ballot means were below 50, the midpoint of the SUS scale, demonstrating that low end range limitation is not an intrinsic characteristic of the scale. A partial replication was performed with a subset of three ballots (best, worst, and midpoint); the results were nearly identical. Further research is required to characterize systems with low SUS scores.

Tips for Usability Practitioners

When using the SUS, consider the following points:

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How Low Can You Go? Is the System Usability Scale Range Restricted?