System Usability Scale (SUS) was designed by John Brooke (DEC, UK) in 1986. It was originally created as a "quick and dirty" usability test on systems like VT100 Terminals.
In a short time, thanks to technology independence and versatility, SUS has been adopted in hardware, consumer software, websites, cell-phones, IVRs, and even yellow-pages tests.
SUS is a 10 item questionnaire with 5 response options (based on the Likert scale) :
- 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.
How to score the SUS
After collecting the data go into the next step to grade usability.
SurveyLab enables you to assign points to any answer choices (test mode), which makes SUS calculation easy.
Step 1. Replace each answer with a number from 0 to 4, using the scheme described below.
for questions 1, 3, 5, 7, 9 the score contribution is :
- Strongly Disagree = 0
- Disagree = 1
- Not sure = 2
- Agree = 3
- Strongly Agree = 4
for questions 2, 4, 6, 8, 10 the score contribution is the opposite :
- Strongly Disagree = 4
- Disagree = 3
- Not sure = 2
- Agree = 1
- Strongly Agree = 0
Step 2. Add values and multiply the total by 2.5. Calculate the mean to find the score. The total score should end up with a range between 0 and 100.
How to interpret SUS
The highest the score the more usable the website is. Values above 68 are considered as good usability.
NOTE. SUS should never be a substitute for good user testing and techniques. It is a low-cost tool that can be used in parallel with user testing to enhance or validate the results.