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System Usability Scale (SUS) Practical Guide for 2024

System Usability Scale (SUS) Practical Guide for 2024
John Bellio
•  23.07.2024

Key Takeaways:

👉 SUS stands for System Usability Scale 

❗ SUS is a standardized post-usability test questionnaire (survey) comprised of 10 Likert-scale questions

✔️ SUS helps measure the perceived ease of use for any systems, ranging from physical to digital products (software, websites, apps etc.)

❌ SUS is not diagnostic; it does not identify the specific factors that make a system usable or unusable

🧠 SUS is a research-backed technique that has been used for decades and provides reliable and valid results

⚖️ To collect SUS score you have participants rate how strongly they agree or disagree with statements on 1-5 scale

💡 While scoring is a bit complicated, its consistency, simplicity and commonality in the industry make it a worthwhile choice to leverage

🧐 Although SUS scores range from 0 to 100, they are not percentages. To evaluate how your product stands in comparison to others, you should examine its percentile ranking

In this article, we’ll provide a practical guide on the system usability scale. We will go over its purpose, briefly mention its origins, provide a guide on how to collect SUS scores and how to interpret results as well as discuss what is a good SUS score.

What is a System Usability Scale? 

The system usability scale (SUS) is a standardized questionnaire (survey) used to measure the perceived usability of a system (product). It provides a score from 0 to 100, where higher scores indicate better usability.

What is a system usability scale questionnaire (survey)?

The SUS questionnaire consists of 10 questions (statements) which respondents answer using a 5-point Likert scale indicating their level of agreement ( 1 = strongly disagree, to 5 = strongly agree). The results provide an overall usability score that demonstrates how easy (or difficult) the system is to use.

If you take a close look at the SUS questions, you will notice that the statements alternate – odd-numbered ones are positive and even-numbered ones are negative. 

This is to prevent respondents from choosing the same answer for all questions and to counteract the acquiescence bias – a natural tendency of people to rather agree with statements than disagree. 

💡 Pro Tip

The SUS survey is designed and should be utilized as a post-test questionnaire, meaning you should give the questionnaire to users after the user test.

Origins of SUS

The System Usability Scale (SUS) was developed by John Brooke in 1984 to provide a tool for measuring the usability of various systems.

Brooke created SUS while working at Digital Equipment Corporation to address the need for a simple yet reliable method to evaluate system usability across different industries.

Over the years, SUS has become a widely adopted standard due to its ease of use, reliability, and validity – in fact, the original paper has over 20,000 citations.

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 following 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 can 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 effectively provide valuable insights to 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 Questions Template

The ten questions in the system usability scale (SUS) are:

  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.

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

Things to remember when creating SUS surveys

  • Do not change the order of the SUS questions
    Maintaining the structure of the SUS questions when performing research is important since changing the order of the questions or scale will affect the scoring when calculating the final results.
  • Do not alter the wording of the questions – altering the wording of the questions can impede comparisons if SUS scores are being collected from other designs and will make it impossible to compare your scores to industry benchmarks.
  • Do not change the wording or order of the Likert scales – as mentioned earlier, changing the wording would make it impossible to compare results with results from other studies. Additionally, altering the order could confuse participants familiar with the traditional Likert scale format, where “strongly disagree” is on the left and agreement increases towards the right.

💡 Pro Tip

Adhering to the standardized SUS questionnaire ensures consistent results.

How to calculate the SUS score?

To score the system usability scale, follow these steps after gathering responses from participants:

1. Subtract one from the user response for odd items (questions)

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.

2. Subtract the user responses from 5 for even-numbered items

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.

3. Sum up the adjusted scores  

Total scores will range from 0 to 40. 

4. Multiply the total score by 2.5. 

This is the system usability score (SUS). Scores will range from zero to 100.

This is how you get the score for one respondent. 

5. Calculate the average score of all respondents

To get the overall score, take the total score of each participant and calculate the average. 

(Overall SUS score = Total score of Repondent 1 + Total score of Respondent 2… + Total score of Respondent N/N) 

You will use your overall SUS score as a benchmark to compare it to past and future SUS scores or with industry averages.

💡 Pro Tip

SUS represents quantitative data, we recommend collecting at least 20 – 30 responses to ensure its significance.

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.

System usability scale scoring example

To better understand how to calculate an SUS score, we prepared 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

SUS score calculator

Here we have prepared a calculator for you – a pre-prepared Excel sheet where you just copy your results, and the function will calculate your SUS score.

👉 Make a copy in Google Sheets

👉 Download it for Excel

How to interpret SUS scores

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. 

A score alone doesn’t indicate whether it’s poor or good; it needs comparison. This can be done by comparing data sets from different products or user groups, or by using norms. Norms are based on data from a large, representative sample to establish percentiles. For instance, a score at the 5th percentile is significantly poorer than one at the 50th or 95th percentile.

Quantifying the User Experience

Jeff Sauro and James R. Lewis (2016) created a curved grading scale using data from 241 usability studies. In this scale, a SUS score of 68 corresponds to a “C” grade, which is average (50th percentile). The top and bottom 15 percentiles correspond to A and F grades, respectively, with further subdivisions for A+, A, A-, and similar breakdowns for B and C grades.

💡 Pro Tip

Check out our article on the Best UX Research Books (According to Our Survey). Besides “Quantifying the User Experience,” which focuses on statistics in user research, you’ll discover other books that will deepen your knowledge of UX.

What is a Good SUS 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 80.8 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

Percentile range

Grade

Rank

84.1 - 100

96 – 100

A+

Best imaginable

80.8 - 84.0

90 – 95

A

78.9 - 80.7

85 – 89

A-

77.2 – 78.8

80 – 84

B+

Excellent

74.1 - 77.1

70 – 79

B

72.6- 74.0

65 – 69

B-

71.1 – 72.5

60 – 64

C+

Good

65.0 - 71.0

41 – 59

C

62.7 - 64.9

35 – 40

C-

51.7 – 62.6

15 – 34

D

Okay

0 - 51.6

0 – 14

F

Poor

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.

How to collect and analyze SUS with UXtweak

1. Create a survey in UXtweak using questions from The System Usability Scale Questions Template in this blog (set up the questions with a 5-point Likert scale).

2. Distribute the study link to participants and gather sufficient responses.

3. Calculate the SUS score in Word using the instructions in the How to calculate the SUS score? section of this article or download the pre-prepared UXtweak excel file in the SUS score calculator section.

4. Compare your results with the ratings on our blog and check the How to interpret SUS scores section to assess your system’s performance.

The Future of the System Usability Scale (SUS)

The System Usability Scale (SUS), has been a cornerstone in usability testing for decades. In a 2018 paper, “The System Usability Scale: Past, Present, and Future,” James R. Lewis emphasizes that SUS remains the most widely used standardized questionnaire for assessing perceived usability. 

He predicts that SUS will continue to be a popular tool for evaluating usability in the foreseeable future.

However, our recent observations suggest a shift.

We’ve noticed that our clients, (various research teams from around the world) don’t use it as much as we expected.

Intrigued, we decided to dig a bit deeper into the data 

Since there are no specific statistics on the current adoption of the SUS, we turned to Google Trends for insights. By examining the interest over time for the topic “System Usability Scale” over the past five years, we aimed to uncover trends in its usage. 

The interactive chart below provides a visual representation of this data:

The numbers on the chart represent search interest relative to the highest point on the chart. A value of 100 signifies peak popularity for the term, while a value of 50 means the term is half as popular.

We calculated the average interest for each year:

Year

Average interest

2019

38.48

2020

38.63

2021

34.48

2022

71.35

2023

63.21

2024

51.61 (up to July)

A notable spike from 34.48 in 2021 to 71.35 in 2022 coincides with Google Trends introducing improvements in data collection, potentially skewing the data. 

Despite this anomaly, the overall trend indicates a decline in interest. From a peak in 2022, interest has dropped to 63.21 in 2023 and further to 51.61 in 2024.

Weekly Patterns

To identify patterns, we also analyzed weekly peaks and declines. Not surprisingly, peaks in interest typically occur early in the year (January) and in the fall (October and November). 

Declines are more frequent in the middle of the year (May to July) and at the year’s end (December). This pattern aligns with the scheduling of major holidays and vacation periods, as well as the typical project cycles observed in UX research teams (researchers often have to wrap up projects before the year’s end and commence new ones at the beginning of the year)

What does this mean? 

While the SUS has undeniably been a critical tool in usability assessment, its usage appears to be waning based on recent trends. The declining interest might reflect shifts towards alternative methods or newer tools that address evolving needs in usability testing. 

Nevertheless, the SUS’s simplicity and effectiveness ensure it will remain a valuable resource in the UX researcher’s toolkit for years to come.

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.

Conduct Usability Testing with UXtweak!

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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.

FAQ: System Usability Scale

What is the system usability scale?

The system usability scale (SUS) is a common questionnaire tool used to measure and understand the perceived usability of a software application or system. It consists of a set of ten questions that participants answer using a Likert scale (1 = strongly disagree, to 5 = strongly agree), and provides an overall usability score that demonstrates how easy (or difficult) an application is to use.

How to calculate the system usability scale score?

To calculate the system usability scale score, gather responses from participants and normalize the scores. For questions 1, 3, 5, 7,  and 9 (the positively oriented questions), subtract 1 from each of the participant’s responses. For questions 2, 4, 6, 8, and 10 (the negatively oriented questions), subtract 5 from each response. Sum up the adjusted score (which should be a number between 0 and 40) and multiply this total score by 2.5. The resulting number is the system usability score (which will be a number between 0 and 100).

What does the system usability scale measure?

SUS scores measure the perceived usability of a system or application. Usability is the user-friendliness of a product, which is determined by how learnable, effective, and efficient a product is to use. High SUS scores indicate better usability, while low SUS scores indicate poor usability for the system being evaluated.

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