When it comes to assessing usability, the system usability scale shines as a tried-and-true research method. In this article, we’ll demystify the dimensions of SUS and explain its purpose, implementation (including scoring), and how to interpret findings to gauge the user-friendliness of your website or application.
Key Takeaways:
➡️ SUS is short for System Usability Scale
❗ SUS is a post-usability test questionnaire tool comprised of 10 Likert-scale questions
✅ SUS helps measure the perceived usability of a website or application
🧠 SUS is a research-backed technique that has been used for decades and provides reliable and valid results
💡 While it may involve a slight learning curve in terms of scoring, SUS’s consistency and commonality in the industry make it a worthwhile choice to leverage
What is a System Usability Scale?
The system usability scale (SUS) is a questionnaire tool used to measure the perceived usability of an application. It consists of a set of ten Likert-scale questions (where answers range from 1 = strongly disagree, to 5 = strongly agree), and provides an overall usability score that demonstrates how easy (or difficult) an application is to use.
The system usability scale was developed in the 1980s, and has been a proven method for valid and reliable research since its inception. It’s often utilized as a post-test instrument, where the SUS questionnaire is given after a usability test is completed.
Participants answer ten Likert-scale questions before results are gathered and scored from 0-100. The Likert scale (pronounced Lick-urt) was created in the 1930s and is used to measure agreement (i.e. whether or not a participant agrees or disagrees with a set of statements).
Benefits of Using a System Usability Scale
The benefits of leveraging the system usability scale are multifactorial. Inherently designed to provide measurement for perceived ease-of-use for a website or application, they can provide deep insight into the follow areas:
- Learnability. How easy a system is to learn. Several questions are included in SUS to better understand how users perceive learnability with the application by exploring how easily users are able to grasp the system.
- Efficiency. How efficient or fast it is to use the system. SUS can provide helpful insight into whether or not users find the application easy or cumbersome to use.
- Satisfaction. How satisfied users are with the system. Though indirect, SUS can help explain whether or not users would regularly enjoy using the application – or if they find it complex.
SUS has a strong record of consistently being a reliable and valid tool for measuring usability. It’s shown to be effective at providing valuable insights with smaller sample size audiences, making them easier to leverage than larger commercial techniques. SUS has also proven to consistently provide valid measurements around perceived usability, meaning any researcher or team can trust the results.
It is important to note, however, that Likert scale questions also open the door to potential biases: acquiescence bias and social-desirability bias. Acquiescence bias refers to people’s tendency to agree with others. Social-desirability bias refers to somones’ desire to report positive views that will be received well by others.
Nevertheless, the system usability scale has withstood the test of time and has established itself as a reliable research method that can be utilized with confidence.
The System Usability Scale Template
The ten questions contained in the system usability scale (SUS) are:
- 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.
Questions 1, 3, 5, 7, and 9 are positively-oriented questions, using wording associated with positive statements, such as “like,” “easy,” “quickly,” and “confident.” Questions 2, 4, 6, 8, and 10 are negatively-oriented questions, where the question and wording have negative associations like “complex,” “inconsistency,” and “cumbersome.” This mixed approach helps capture a comprehensive perspective of a user’s experience with the application.
As participants go through the questionnaire, they rank their responses on a scale of one to five (Likert scale), where 1 = “Strongly disagree”, and 5 = “Strongly agree”. For example:
Question: I think that I would like to use this system frequently.
Possible answers:
Strongly disagree | Disagree | Neither agree nor disagree | Agree | Strongly agree |
1 | 2 | 3 | 4 | 5 |
Maintaining the structure of the SUS questions when performing research is important, since changing the order of the questions affects the scoring when calculating final results. Additionally, altering the wording of the questions can impede comparisons if SUS scores are being collected from other designs. Adhering to the standardized SUS questionnaire ensures consistent results.
System Usability Scale Scoring
To score the system usability scale, follow these steps after gathering responses from participants:
Step 1: Take each positively-oriented question score (i.e., for questions 1, 3, 5, 7, and 9) and subtract one from the participant’s answer. Answers will range from zero to four.
Step 2: Take each negatively-oriented question score (i.e., for question 2, 4, 6, 8, and 10) and subtract the participant’s answer from five. Answers will range from zero to four.
Step 3: Sum up the adjusted scores for all items to obtain a total score. Total scores will range from zero to 40.
Step 4: Multiply the total score by 2.5. This is the system usability score (SUS). Scores will range from zero to 100.
Scoring the SUS brings a bit of complexity, and is one of the impediments for using the system. However, because the system usability scale has demonstrated its value experimentally for decades, there is a wide-range of industry data available to benchmark results and understand them within the context of the competition. This makes the learning curve to operating the system worthwhile.
Calculating SUS scores
To understand how to calculate a SUS score, let’s consider an example.
Let’s imagine that a participant completed the SUS questionnaire following a usability test. Their responses are below:
Question Number | Participant Responses (Ranked One to Five) |
1 | 5 |
2 | 5 |
3 | 4 |
4 | 1 |
5 | 3 |
6 | 1 |
7 | 3 |
8 | 2 |
9 | 5 |
10 | 1 |
Step 1: Take each positively-oriented question score (questions 1, 3, 5, 7, and 9) and subtract one from the participant’s answer. Answers will range from zero to four.
Example:
- Participant score: 5 (Strongly agree)
- Subtract one from the participant’s answer to get four (5 – 1 = 4)
Question Number (Positively Oriented) | Participant Response (Ranked One to Five) | Adjusted Score (Response Minus One) |
1 | 5 | 4 |
3 | 4 | 3 |
5 | 3 | 2 |
7 | 3 | 2 |
9 | 5 | 4 |
Step 2: Take each negatively-oriented question score (questions 2, 4, 6, 8, and 10) and subtract the participant’s answer from five. Answers will range from zero to four.
Example:
- Participant score: 5 (Strongly agree)
- Subtract the participant’s answer from five to get zero (5 – 5 = 0)
Question Number (Negatively Oriented) | Participant Response (Ranked One to Five) | Adjusted Score (Response Minus One) |
2 | 5 | 0 |
4 | 1 | 4 |
6 | 1 | 4 |
8 | 2 | 3 |
10 | 1 | 4 |
Step 3: Sum up the adjusted scores for all items to obtain a total score. Total scores will range from zero to 40.
Question Number | Participant Response | Adjusted Score |
1 | 5 | 4 |
2 | 5 | 0 |
3 | 4 | 3 |
4 | 1 | 4 |
5 | 3 | 2 |
6 | 1 | 4 |
7 | 3 | 2 |
8 | 2 | 3 |
9 | 5 | 4 |
10 | 1 | 4 |
Total Adjusted Score | 30 |
Step 4: Multiply the total score by 2.5. This is the system usability score (SUS). Scores will range from zero to 100.
Example:
30 x 2.5 = 75 (SUS)
SUS Score | 75 |
What is a Good SUS Score?
When determining the strength of an overall SUS score, it’s important to note that the final 0-100 SUS score is not equivalent to a percentage score. Jeff Sauro, PhD, has found that the average SUS score is 68. Therefore, a SUS score of 70 doesn’t mean the same thing as scoring 70% on an exam – because the 70 point SUS score is actually above average.
Additionally, a SUS score of 80.3 is actually in the top 10% of all SUS scores, which would be similar to scoring an “A” on a test (even though the score is 80 out of 100).
The below table normalizes several ranges of SUS scores by ‘grade’ in order to help better rank usability.
SUS Score | Grade | Rank |
> 80.3 | A | Excellent |
68 – 80.3 | B | Good |
62 – 67 | C | Okay |
51 – 62 | D | Poor |
< 51 | F | Awful |
When it comes to interpreting SUS scores, it’s important to remember that there are also many other acceptable ways to provide meaningful comparisons – such as with percentiles, acceptability, and NPS.
Is the System Usability Scale the right tool for you?
If you’re seeking to better understand how to measure the perceived usability of your website or application, leveraging the system usability scale can be a valuable approach.
However, since SUS is a post-test questionnaire, it’s important to set up and conduct usability tests beforehand. UXtweak offers a wide array of testing options for testing your website or mobile application, suitable whether you’re in early prototype stages or have an established product already on the market. You can explore options and get started with a free trial today.
Conclusion
In summary, SUS scores are a helpful measurement to learn whether or not users find your website or app easy-to-use and efficient. Additionally, it can provide a global measure of satisfaction and help provide deeper insight into particular dimensions of usability.
When it comes to today’s competitive landscape, enhancing efficiency, intuitiveness, and usability has become an order qualifier. Begin your journey to better and more efficient systems by partnering with UXtweak and leveraging the power of SUS.
