Abstract. Since its introduction in 1986, the 10-item System Usability Scale (SUS) has been assumed to be unidimensional. Factor analysis of two independent SUS data sets reveals that the SUS actually has two factors -Usable (8 items) and Learnable (2 items -specifically, Items 4 and 10). These new scales have reasonable reliability (coefficient alpha of .91 and .70, respectively). They correlate highly with the overall SUS (r = .985 and .784, respectively) and correlate significantly with one another (r = .664), but at a low enough level to use as separate scales. A sensitivity analysis using data from 19 tests had a significant Test by Scale interaction, providing additional evidence of the differential utility of the new scales. Practitioners can continue to use the current SUS as is, but, at no extra cost, can also take advantage of these new scales to extract additional information from their SUS data. The data support the use of "awkward" rather than "cumbersome" in Item 8.

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When designing questionnaires there is a tradition of including items with both positive and negative wording to minimize acquiescence and extreme response biases. Two disadvantages of this approach are respondents accidentally agreeing with negative items (mistakes) and researchers forgetting to reverse the scales (miscoding).The original System Usability Scale (SUS) and an all positively worded version were administered in two experiments (n=161 and n=213) across eleven websites. There was no evidence for differences in the response biases between the different versions. A review of 27 SUS datasets found 3 (11%) were miscoded by researchers and 21 out of 158 questionnaires (13%) contained mistakes from users.We found no evidence that the purported advantages of including negative and positive items in usability questionnaires outweigh the disadvantages of mistakes and miscoding. It is recommended that researchers using the standard SUS verify the proper coding of scores and include procedural steps to ensure error-free completion of the SUS by users.Researchers can use the all positive version with confidence because respondents are less likely to make mistakes when responding, researchers are less likely to make errors in coding, and the scores will be similar to the standard SUS.

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