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What is the SUS scale and how to use it to measure usability?

Stefano Serafinelli
March 8, 2024

The SUS scale (System Usability Scale) is a usability evaluation tool used to measure the ease of use of a system, product, or service. It was developed by John Brooke in 1986 and has become one of the most widely used measures in the field of user experience.

It consists of a brief questionnaire that addresses various aspects of the user experience, such as usability, efficiency, and overall satisfaction. Through it, we obtain a quantitative measure of the system's usability perceived by its users.

How is the SUS scale used?

The SUS scale uses a standard questionnaire consisting of 10 questions. This is provided to users of a system who have direct experience with it, ideally immediately after using it so they can evaluate their usage experience promptly and accurately.

Each question will be scored on a Likert scale from 1 to 5, with 1 being "completely disagree" and 5 being "completely agree".

Using standard questions allows the results to be compared with other systems, products, or with the same system over time.


The 10 questions of the SUS questionnaire are as follows:

  1. I think that I would like to use this system frequently.
  2. I found the system unnecessarily complex.
  3. I thought the system was easy to use.
  4. I think that I would need the support of a technical person to be able to use this system.
  5. I found the various functions in this system were well integrated.
  6. I thought there was too much inconsistency in this system.
  7. I would imagine that most people would learn to use this system very quickly.
  8. I found the system very cumbersome to use.
  9. I felt very confident using the system.
  10. I needed to learn a lot of things before I could get going with this system.

How to measure the results of a SUS scale

Obtaining the corresponding SUS metric requires some mathematical calculations and following the steps below:

  • For odd-numbered questions (1, 3, 5, 7, 9), subtract 1 from the user's response.
  • For even-numbered questions (2, 4, 6, 8, 10), subtract the user's response from 5.
  • Add up all the adjusted scores.
  • Multiply the sum by 2.5.

The formula can be expressed mathematically as:

SUS=((Q1−1)+(5−Q2)+(Q3−1)+(5−Q4)+(Q5−1)+(5−Q6)+(Q7−1)+(5−Q8)+(Q9−1)+(5−Q10))×2.5

In this formula, the variables from Q1 to Q10 are the responses to the 10 questions.

Once we have calculated the SUS for each participant, we need to average all the results to obtain the total SUS for our system.

The good news is that it is not necessary to manually calculate the SUS. Numerous online tools are available to automatically and quickly calculate the SUS.

 

We also have one that you can download and use freely

Download our SUS calculator template ->

What is a good SUS score?

The SUS scale returns a score from 0 to 100, where a higher score corresponds to better perceived usability by the study participants.

Jeff Sauro, a UX research expert with a special focus on quantitative and statistical techniques, has been reviewing SUS results from over 5000 users across 500 different studies and has concluded that the average SUS score of all analyzed systems is 68 out of 100.

Additionally, Sauro identifies different ranges of perceived usability based on the obtained SUS score, rating it from F (poor usability) to A (excellent usability).
For reference, a score of over 78.9 is needed to receive an A grade, which means being among the top 15%.

SUS score
Grade
Percentile
78,9 - 100
A
Top 15%
72,6 - 78,8
B
Top 35%
62,7 -72,5
C
Top 65%
51,7 - 62,6
D
Top 85%
0 - 51,6
F
-

Our SUS calculator also provides, along with the score, the corresponding rating, allowing us to better assess how our system compares to the market.

Another way to interpret SUS results is based on their acceptability compared to the average. An acceptable result would score above 70, while an unacceptable result would score below 50. Results falling between 50 and 70, roughly corresponding to a grade from C to D, are marginally acceptable

It's important to note that these ranges are just a guideline and that the results of the SUS scale should be interpreted in the context of each particular system or product.

Limitations of the SUS scale

Although the SUS scale is a very useful tool for obtaining quantitative and comparable metrics, it's important to note that it also has limitations.

Firstly, the SUS scale measures perceived usability, which, at times, may differ from actual usability.
For example, a user might consider their experience with the product to be very satisfactory and without much difficulty, while in reality, they have experienced many usability issues that they may not have noticed.
For instance, they might exit an e-commerce platform believing they have completed a purchase correctly, when in fact they still needed to confirm the final step.

In such cases, we shouldn't solely rely on the good SUS score we obtained, but rather pay particular attention to the issues experienced by the user.

Related to the above, it's important to consider that SUS is a quantitative technique that only provides us with a metric and does not inform us about the actual problems users encounter with our system. Like all quantitative techniques, it's necessary to combine them with qualitative ones to delve into our users' difficulties and, above all, their causes.

Lastly, like all quantitative metrics, it's necessary to consider that the sample size of our study influences the accuracy of the data. If we calculate SUS with a small number of users, the margin of error is likely to be very wide, and the obtained result may deviate considerably from the actual data.

In conclusion

The SUS scale is a standard tool that allows us to evaluate the perceived usability of a system by its users, providing a score from 0 to 100, where 100 represents the best possible result. It is a very useful tool for comparing the system's evolution over time and also for making comparisons with similar systems from competitors.

However, it needs to be complemented with qualitative techniques to identify specific improvement opportunities and obtain clear indications on how to enhance our system.

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