System Usability Scale
Have you heard about System Usability Scale (SUS)? Neither had I until a few years ago. Let me explain what it is and why I think it's an important tool for anyone working with usability and user experience.
Written by Magnus Hjelm
What is SUS?
The SUS, System Usability Scale is a way to measure the usability of a system. It was first developed by John Brooke in 1986. The SUS has become the most commonly used method for measuring the usability of a system, software, website or app. The tool is designed to be very easy to use and is effective in both small and large scale user studies.
How does it work?
SUS is a 10 question survey based on a 5 point Likert scale ranging from strongly disagree to strongly agree. The questions are numbered from 1 to 10 and the person taking the test is asked to indicate their level of agreement with each question.
How to calculate SUS score?
When you have the result from your test participants it’s time to calculate the SUS score. The method for calculating SUS score can look a bit complex, so let’s break it down step by step. We use the following response from one of the participants in our test:
- Add up the total score for all odd-number questions (4+5+3+4+3 = 19).
- Add up the total score for all even-number questions (2+1+3+1+1 = 8)
- Subtract 5 from the odd number (19 - 5 = 14)
- Take the even number and subtract that from 25 (25 - 8 = 17)
- Add the two new numbers (14 + 17 = 31)
- Multiply with 2.5 (31 x 2.5 = 77.5)
This will give you a number between 0 and 100, the higher the better. In our case we get a SUS score of 77.5. So what does this mean?
Interpreting SUS score
First off, the score is not a percentage, it’s just a score where 100 is the maximum result. A score of 50 doesn’t mean an average result – instead research has shown that a score of 68 is the average.
To make the score easier to present and talk about it can be converted into a grade. There are different ways to do this conversion and if you google you will find many different kinds of scales and grades. I prefer this one with a grade F to A.
A personal story about SUS
I have worked in the UX field well over 10 years, but I only learnt about SUS a few years ago. Since then it has become an important part of my tool belt.
A SUS score by itself doesn’t give me much value. Instead SUS needs to be used in conjunction with usability testing and user research activities. I need to talk to and observe users to understand how I can improve my solutions.
For me SUS scores becomes valuable when I communicate my findings with management and stakeholders. The result from a usability test presented together with a SUS score is powerful combination.
When doing user research, interviews or usability testing it is not much extra effort to let the participant also fill out the SUS questionnaire, either digital or on paper. Answering the questionnaire usually doesn’t take much more than 2 minutes, often faster.
Ways to measure SUS
You can measure SUS in many different ways. A straightforward way is to hand out a questionnaire in paper format to the participant after you have conducted your interview or usability test.
If you frequently do user research activities and measure SUS, you might want a more structured way. For this purpose, ThinkLoud SUS offers a unique solution. Within the same tool, you and your team can set up, measure and, analyze your SUS.