System Usability Scale (SUS): for measuring usability
The System Usability Scale (SUS) is a questionnaire that measures the usability or usability of a website or app, taking into account the context in which the product is used. You can read more about usability and usability testing in this article: what is usability testing?
In this article you will find out where the system usability scale comes from, we explain how to interpret the results.
The origins of the System Usability Scale (SUS)
The System Usability Scale (SUS) was developed and put into use well before the breakthrough of the internet. In 1986, John Brooke developed the questionnaire and over 30 years later it is still in wide use. How so?
Before John Brooke developed the System Usability Scale, he noted that measuring the usability of a product is not an exact science - and largely depends on the context in which the product is used. Something that is still valid today!
Take, for example, a logistics model designed to maintain medicine stocks in the EU. There you look at different factors than when measuring the usability of an Albert Heijn's self-scanner.
What was missing at the time was an efficient tool to gain insight into the usability of your own system, but at the same time compare it to other systems in the market.
That void was filled by the System Usability Scale. A questionnaire with ten questions measuring the usability of a product, but taking into account the context in which the product is used.
This made it possible to measure usability uniformly and compare it with other products, companies and industries. Very useful!
Advantages System Usability Scale
- Uniform output and benchmark. The biggest advantage is that the SUS expresses usability on a scale of 1 to 100, allowing you to benchmark the results against other products, the competition or the industry average.
- Reliable results. The System Usability Scale has been tested several times and provides reliable results, even on smaller samples.
- Quick overview of usability. Because the SUS questionnaire consists of only ten questions, it takes little time to complete and gives you a quick indication of usability.
The System Usability Scale Questionnaire
Respondents are asked to rate the ten statements below using the Likert scale. There are five answer options here, ranging from completely disagree to completely agree. Download the SUS questionnaire for free.Download SUS Questionnaire
Calculate SUS Score
Follow the steps below to calculate the SUS Score. Take into account the points associated with each answer:
1 point - Completely Disagree
2 points - Disagree
3 points - Neutral
4 points - Agree
5 points - Totally agree
Step 1 - Calculate Odd Number Score - (X-1)
The first step is to calculate the score for all odd numbers. You do this by looking at the number of points scored and subtracting 1 point (X-1).
Example: When someone answers "Agree" to the first question, it is worth four points (X = 4, see table). From this you get a point, making the score for that question 3 points (X-1).
Step 2 - Calculate score even numbers - (5 - X)
The second step is to calculate the score for all even numbers. You do this by looking at the number of points scored and subtracting it from 5 (5-X).
Example: When someone disagrees with the first question, it is worth two points (X = 2, see table). Then you subtract two from five, making the score for that question 3 points (5-X).
Step 3 - Calculate the SUS Score - (Score step 1 + score step 2) * 2.5
In this step you add all points from steps 1 and 2 together. You then multiply the total score by 2.5. The answer you get is the SUS score!
Example: Assume the total number of points from step 1 is 15 (3 + 4 + 2 + 4 + 2 = 15). And in step 2 10 (2 + 4 + 3+ 1 + 0 = 10). Add 15 and 10 together to get 25 points. You multiply this by 2.5 (15 + 10) * 2.5 = 62.5. The SUS score is then 62.5.
Step 4 - Interpreting the results
If you have calculated the SUS score using the above method, you get a score (no percentage!) Between 1 and 100. In addition, a score of more than 80.3 means that you are in the top 10%, a score of above 68 in the top 30% and from 68 in the top 50%.
80.3 or above - Excellent
68 - 80.3 - Good
68 - OK
51 - 68 - Below Average
51 or below - Poor
Side note on System Usability Scale
Although the System Usability Scale is well-established among many UX researchers, you would do well to take the following into account.
- Not an all-purpose usability metric. Like all other usability metrics, the score by itself does not tell the whole picture. Consider making use of other usability metrics such as the Net Promoter Score, Single Ease Question or the SUPR-Q to complement the System Usability Scale.
- Statistically (in)significant. When conducting usability research, small respondent numbers are often used (on average 5 per device type). This allows you to make a statement about how this group of respondents experienced the website, but does not mean that this score applies to all website visitors.
- Limited depth. Although you can infer from the SUS score how respondents experienced the website or app, it remains a guess as to why this is the case. Therefore, qualitative analysis of usability tests will always remain necessary.