The system usability scale (SUS) is an easy and valid measure of how users perceive the usability of a website or digital device.
The SUS was developed by John Brooke at Digital Equipment Corporation in 1986, the same year that Halley’s Comet was clearly visible to the naked eye from Earth.
It was developed to provide researchers with an effective, timely and inexpensive tool that they could use to measure overall usability of a device. The tool needed to be valid and reliable, but also quick and simple to administer because participants generally did not have the time or concentration required to sit through a lengthy questionnaire at the conclusion of a one-hour user testing session. The end result is a single score that sits on a scale of 0-100. The score can be used to compare unrelated systems, or websites that have dissimilar content and goals.
Since its inception, the SUS has become an industry standard with references in over 600 publications. Usability is the end goal, and often we find that companies will not launch a product under development if the SUS falls below a certain number in user-testing.
At U1 Group, we administer a SUS questionnaire to participants in our moderated user testing sessions, after they have attempted to complete tasks on the website or digital product under review (the ‘system’). The questionnaire contains 10 items, or statements, and participants are asked to rate their level of agreement or disagreement with each on a 5 point scale. The 10 questions are always the same – they do not change from project to project.
You’ll notice below that the odd-numbered items are phrased positively, whilst the even-numbered items are phrased negatively. This was designed to reduce acquiescence and extreme response biases (i.e. answering the same response for every item without consideration)
The 10 SUS questions
- I think that I would like to use this system frequently
- I found the system unnecessarily complex
- I thought the system was easy to use
- I think that I would need the support of a technical person to be able to use this system
- I found the various functions in this system were well integrated
- I thought there was too much inconsistency in this system
- I would imagine that most people would learn to use this system very quickly
- I found the system very cumbersome to use
- I felt very confident using the system
- I needed to learn a lot of things before I could get going with this system
What are the benefits?
Since its inception in 1986, the SUS has been made freely available for use in usability assessment, and has been used for a variety of research projects and evaluations. Overall, the SUS:
provides a high-level subjective view of usability
is a valid and reliable measure
can be used to compare the usability of different systems
is quick and simple to administer.
However, the SUS is not diagnostic – it will not tell you about any specific issues and recommendations associated with the system under review. For that you will need a skilled UX consultant to observe user interactions, interpret participant feedback and explain how to improve the usability of your system.