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Determining What Individual SUS Scores Mean: Adding an Adjective Rating Scale

Aaron Bangor, Philip Kortum, and James Miller

Journal of Usability Studies, Volume 4, Issue 3, May 2009, pp. 114-123

Article Contents


Methods

The SUS is composed of ten statements, each having a five-point scale that ranges from Strongly Disagree to Strongly Agree. There are five positive statements and five negative statements, which alternate. While the SUS has been demonstrated to be fundamentally sound, our group found that some small changes helped participants complete the SUS. First, a short set of instructions were added that reminded them to mark a response to every statement and not to dwell too long on any one statement. Second, the term cumbersome in the original Statement 8 was replaced with awkward. (This same change was independently made by Finstad, 2006.) Finally, the term system was changed to product, based on participant feedback. The current SUS form being used in our laboratories is shown in Figure 1.

Figure 1. Our current version of the System Usability Scale (SUS), showing the minor modifications to the original Brookes instrument

Figure 1. Our current version of the System Usability Scale (SUS), showing the minor modifications to the original Brookes instrument

We have used this version of the SUS in almost all of the surveys we have conducted, which to date is nearly 3,500 surveys within 273 studies. It has proven to be a robust tool, having been used many times to evaluate a wide range of interfaces that include Web sites, cell phones, IVR, GUI, hardware, and TV user interfaces. In all of these cases, participants performed a representative sample of tasks for the product (usually in formative usability tests) and then, before any discussion with the moderator, completed the survey. Table 1 lists survey count and mean scores by user interface type.

Table 1. Summary of SUS Scores by User Interface Type

Table 1. Summary of SUS Scores by User Interface Type

The overall mean of about 70 has remained constant for some time now. It is slightly lower than the median score of 70.5, which reflects the negative skew to the set of study mean scores. In fact, fewer than 5% of all studies have a mean score of below 50 (although 18% of surveys fall below a score of 50). The quartile breakdown of study mean scores is shown in Table 2.

Table 2. Quartiles for SUS Study Mean Scores (n=273 studies)

Table 2. Quartiles for SUS Study Mean Scores (n=273 studies)

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