System Usability Scale (SUS)

System Usability Scale (SUS) is one of the most used usability assessment questionnaires, and it was developed in 1986 by John Brooke.

The SUS score can tell the usability performance in the aspects of effectiveness, efficiency, and overall ease of use. It consists of a series of ten Likert scale questions and produces a score between 0-100 (note that the scale is not equivalent to a %-score). SUS is used as a post-test questionnaire and has a built-in benchmark.

Benefits of SUS:

• SUS is quick to use and interpret, and part of the standard Preely questions
• SUS is a cheap solution since not much effort goes into using it
• SUS is tested and reliable
• It gives you numbers to convey the usability of your concept or products to stakeholders and upper management

So all in all, SUS is a great metric to include in your usability and UX testing framework!

Three of the SUS questions in Preely.

One of the significant advantages of SUS is that it has been validated over many years and there is a large amount of industry-wide data available to help benchmark your score and understand it in the context of your peers and competitors.

Another benefit is that SUS correlates strongly with NPS (read more about NPS). They provide different data and NPS is not as sensitive to usability and UX-focused concerns as SUS.

If you want to dig deeper into this, SUS and NPS scores have a significant correlation of r=.61 (promoters had an average SUS of 82 and detractors had an average SUS of 67).

How to calculate the SUS score

In Preely you can add System Usability Scale to your test. You find it under ‘Add question’ when you create your test. The platform automatically calculates a SUS score for each participant and an overall SUS score for the test.

Going into a bit of detail about the calculation of SUS, the total SUS score is 100 and each question has a weight of 10 points.
All odd-numbered questions are phased positive, hence all ‘strongly agree’ responses are given a maximum point of 10 for each question. All ‘strongly disagree’ is given a minimum point of 0. We then subtract 1 from each (5 in total) to ensure that the minimum is 0. Then we multiply by 2.5 to ensure that the maximum is 10 for each question.
All even-numbered questions are phased negative, hence all ‘strongly agree’ responses are given a minimum point which is 0 for each question. All ‘strongly disagree’ are given a maximum point of 10. We then subtract the points from 5 (25 in total) to ensure that the minimum is 0. Then we multiply by 2.5 to ensure that the maximum is 10 for each question.

Confused – no worries, the platform does everything for you!

How to interpret SUS

Remember SUS is not equivalent to %-score. The average SUS score is 68. This means that a score of 68 will put you at the 50th percentile. If your level of usability should be in the top 10%, you need a score of 80.3 or higher. We have added a guideline for the interpretation below.

Guideline on the interpretation of SUS score
Source