Measuring usability with the System Usability Scale

A quick and dirty questionnaire for measuing usability

Joshua

3 minute read

System Usability Scale

Usability and user experience are difficult to measure quantitatively. Often, measuring improvements (or regressions) in usability is primarily achieved through monitoring a range of interrelated metrics such as average session length, goal completion rates, defects raised or churn. These metrics, however, can all be influenced both positively and negatively by many factors - not just usability. One way of achieving direct feedback on the usability of a product, and thus being able to more directly measure it, is to use a questionnaire.

The System Usability Scale (SUS) is a self described quick and dirty method for assessing the usability of a system via a simple set of 10 questions. The SUS was was developed by John Brooke in 1986 to help measure the usability of electronic office systems. You can read the original paper and a follow up retrospective by Brooke - both worth a read.

System Usability Scale Questionnaire

The questions are as follows:

  1. I think that I would like to use this system frequently.
  2. I found the system unnecessarily complex.
  3. I thought the system was easy to use.
  4. I think that I would need the support of a technical person to be able to use this system.
  5. I found the various functions in this system were well integrated.
  6. I thought there was too much inconsistency in this system.
  7. I would imagine that most people would learn to use this system very quickly.
  8. I found the system very cumbersome to use.
  9. I felt very confident using the system.
  10. I needed to learn a lot of things before I could get going with this system.

Scoring the System Usability Scale Questionnaire

The scoring method is outlined in the paper - briefly each question is scored 0 - 4. For odd questions the score is the scale position minus 1. For even questions, the contribution is 5 minus the scale position. The total score is multiplied by 2.5 to obtain the overall result.

Interactive System Usability Scale Questionnaire

So you can see it in action, you can use the System Usability Questionnaire here. Complete the questions and then you’ll see the results. If you want, you can direct users of your site to the questionnaire - once the score has been generated, they can mail you the results.

Summary

I’m a fan of anything that helps measure improvements, and so I’ve used the SUS in the past as a way of benchmarking a products usability prior to making improvements. With a periodic assessment, you can see how changes that have been made incrementally (or perhaps rapidly) change the percieved usability of the site.

A questionnaire cannot (and should not) replace other usability and user experience tasks from your day to day life. However, as a standardised test, the SUS can give you another helpful data point in moving your product forwards. Its also been observed that higher SUS score correlate with higher Net Promoter Score (NPS), so focusing on usability is always likely to provide some pay off.



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