Ever wondered how companies measure how user-friendly their products really are? Enter the System Usability Scale (SUS) — a simple yet powerful tool that turns subjective user experiences into clear, actionable data. Let’s dive into what makes SUS a gold standard in usability testing.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has since become one of the most widely used tools in usability evaluation across industries ranging from software development to healthcare technology.
Origins and Development of SUS
The SUS was first introduced in 1986 as a quick and reliable way to evaluate usability without requiring complex methodologies. At the time, usability testing was often time-consuming and expensive, involving detailed observational studies and expert reviews. Brooke sought a lightweight alternative that could be administered quickly and still yield meaningful results.
Brooke’s original research aimed to create a scale that was both technology- and application-neutral.The SUS was designed to be easy to administer and score, making it accessible even to non-specialists.It gained traction in the 1990s and early 2000s as digital interfaces became more prevalent..
“The System Usability Scale is not a measure of functionality or performance, but of the user’s perception of how easy or difficult a system is to use.” — John Brooke
Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5).The statements alternate between positive and negative phrasing to reduce response bias.After scoring, the final SUS score ranges from 0 to 100, with higher scores indicating better perceived usability..
- Odd-numbered items are positively worded (e.g., “I thought the system was easy to use”).
- Even-numbered items are negatively worded (e.g., “I found the system unnecessarily complex”).
- The scoring algorithm adjusts for reverse scoring on even-numbered items.
Why the System Usability Scale Matters in UX Design
In the world of user experience (UX) design, understanding how users perceive a product is just as important as how it functions. The system usability scale provides a standardized metric that helps teams quantify subjective feedback, enabling data-driven decisions in design iterations.
Quantifying User Experience
One of the biggest challenges in UX is measuring something inherently subjective. While metrics like task completion time or error rates are objective, they don’t capture the user’s overall impression. The system usability scale bridges this gap by converting qualitative feelings into a numerical score.
- SUS scores allow for benchmarking across different versions of a product.
- They enable comparisons between competing products or interfaces.
- Teams can track usability improvements over time using SUS as a KPI.
Supporting Agile and Iterative Design
In agile development environments, where rapid prototyping and frequent releases are the norm, the system usability scale fits seamlessly into the workflow. Because it’s quick to administer and analyze, SUS can be used after each sprint to evaluate new features or design changes.
- Designers can gather SUS data from small user groups (as few as 5–8 participants).
- Results are available within minutes of data collection.
- Teams can prioritize usability fixes based on SUS trends.
How to Administer the System Usability Scale
Administering the system usability scale correctly is crucial to obtaining reliable and valid results. While the process is straightforward, attention to detail ensures that the data collected is meaningful and comparable.
Best Practices for Administering SUS
To get the most accurate results, follow these best practices when using the system usability scale:
- Administer the SUS immediately after the user completes a set of representative tasks.
- Ensure participants have had enough interaction with the system to form an opinion.
- Use a consistent format (paper, online survey, or interview) across all participants.
- Avoid leading questions or influencing responses during administration.
“The validity of the SUS score depends heavily on the context in which it’s used. It should follow a realistic user session, not a forced or artificial one.” — UX Research Guidelines, Nielsen Norman Group
Common Mistakes to Avoid
Despite its simplicity, there are several pitfalls that can compromise the integrity of SUS results:
Administering SUS too early: If users haven’t interacted enough with the system, their responses may not reflect true usability.Using SUS in isolation: While SUS provides a great high-level view, it doesn’t explain why users feel a certain way.Always pair it with qualitative methods like interviews or think-aloud protocols.Misinterpreting the score: A score of 68 is average, not poor.
.Many teams panic over scores they think are low, when in fact they’re within the normal range.Scoring and Interpreting the System Usability Scale
One of the most appealing aspects of the system usability scale is its straightforward scoring method.Despite its simplicity, the scoring algorithm is robust and produces a reliable metric that can be interpreted with confidence..
Step-by-Step Scoring Guide
Here’s how to calculate a SUS score manually:
- For each odd-numbered item, subtract 1 from the user’s response (so a score of 1 becomes 0, 2 becomes 1, etc.).
- For each even-numbered item, subtract the user’s response from 5 (so a 5 becomes 0, 4 becomes 1, etc.).
- Sum all the adjusted scores.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if a user’s adjusted sum is 27, the final SUS score is 27 × 2.5 = 67.5.
What Do SUS Scores Mean?
Interpreting SUS scores requires context, but general benchmarks have been established through years of research:
- Below 50: Poor usability — significant redesign needed.
- 50–65: Below average — usability issues likely present.
- 66–75: Average — acceptable but room for improvement.
- 76–85: Good — above average, users find it easy to use.
- 85–100: Excellent — top-tier usability, rare in practice.
A landmark study by Sauro and Lewis (2006) analyzed over 500 SUS scores and found the average to be around 68, which serves as a useful benchmark for comparison.
Advantages of Using the System Usability Scale
The enduring popularity of the system usability scale is no accident. Its widespread adoption is due to a combination of practical benefits that make it ideal for both academic research and industry applications.
Reliability and Validity
Despite its brevity, the system usability scale has demonstrated strong psychometric properties. Numerous studies have confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
- SUS has high internal consistency, with Cronbach’s alpha typically above 0.9.
- It correlates well with other usability metrics like task success and user satisfaction.
- It performs consistently across different languages and cultures when properly translated.
Speed and Simplicity
In fast-paced development cycles, time is a critical factor. The system usability scale can be completed in under 5 minutes, making it one of the fastest usability assessment tools available.
- Users can complete the questionnaire without extensive training.
- Data can be analyzed manually or with automated tools.
- Results are easy to communicate to stakeholders who may not be usability experts.
Limitations and Criticisms of the System Usability Scale
While the system usability scale is a powerful tool, it is not without limitations. Understanding these weaknesses helps practitioners use SUS more effectively and avoid over-reliance on a single metric.
Lack of Diagnostic Detail
One of the most common criticisms of the system usability scale is that it doesn’t tell you why a system is perceived as usable or not. A low score indicates a problem, but not where it lies.
- SUS cannot identify specific usability issues like navigation problems or unclear labels.
- It doesn’t differentiate between learnability, efficiency, and satisfaction — all of which are part of usability.
- Teams must supplement SUS with other methods like heuristic evaluation or user interviews.
Cultural and Linguistic Sensitivity
Although SUS has been translated into dozens of languages, direct translations may not always capture the intended meaning. Cultural differences in how people interpret Likert scales can also affect results.
- Some cultures tend to avoid extreme responses, leading to compressed scoring ranges.
- Idiomatic expressions in the original English version may not translate well.
- Best practice is to validate translated versions through pilot testing.
Practical Applications of the System Usability Scale
The system usability scale is not just a theoretical tool — it’s actively used in real-world scenarios across multiple industries. From software development to medical devices, SUS helps organizations improve their products and services.
Software and App Development
In the tech industry, SUS is frequently used to evaluate mobile apps, web platforms, and enterprise software. Companies like Google, Microsoft, and Spotify have reportedly used SUS or similar metrics in their UX research.
- Used to compare different design prototypes before launch.
- Helps prioritize bug fixes and feature enhancements.
- Integrated into continuous usability monitoring programs.
Healthcare and Medical Devices
In healthcare, where usability can directly impact patient safety, the system usability scale is a critical tool. Regulatory bodies like the FDA encourage usability testing for medical devices, and SUS is often included in these evaluations.
- Used to assess the usability of electronic health record (EHR) systems.
- Helps ensure that medical devices are intuitive for healthcare professionals.
- Supports compliance with human factors engineering standards.
Alternatives and Complements to the System Usability Scale
While the system usability scale is widely respected, it’s not the only usability assessment tool available. Depending on the context, other questionnaires or methods may be more appropriate — or can be used alongside SUS for a more complete picture.
Usability Questionnaire (UMUX) and UMUX-Lite
The Usability Metric for User Experience (UMUX) is a four-item questionnaire based on the ISO definition of usability. UMUX-Lite, a two-item version, is even shorter than SUS and highly correlated with it.
- UMUX focuses on usability as effectiveness, efficiency, and satisfaction.
- UMUX-Lite is ideal for situations where survey length is a concern.
- Both are good alternatives when SUS feels too long.
Net Promoter Score (NPS) and SUS
Some organizations combine the system usability scale with the Net Promoter Score to get both usability and loyalty insights. While NPS measures willingness to recommend, SUS measures perceived ease of use.
- High SUS scores often correlate with high NPS, but not always.
- Using both provides a more holistic view of user experience.
- Caution: NPS is a business metric, not a usability one.
Future of the System Usability Scale in Digital Experience
As technology evolves, so do the ways we measure usability. The system usability scale has proven adaptable over decades, but new challenges — from AI-driven interfaces to immersive VR environments — are pushing the boundaries of traditional usability assessment.
Adapting SUS for Emerging Technologies
While SUS was designed for traditional software interfaces, researchers are exploring its applicability to newer domains:
- Studies have successfully applied SUS to evaluate voice assistants like Alexa and Google Assistant.
- It’s being used in AR/VR applications, though some suggest modifications for spatial interaction.
- AI-powered chatbots are also being assessed using SUS, despite their non-traditional interface.
Integration with Automated UX Analytics
The future of usability measurement may lie in combining subjective tools like SUS with objective behavioral data. Tools that track mouse movements, click paths, and session duration can now be correlated with SUS scores to create richer insights.
- Machine learning models are being trained to predict SUS scores from behavioral patterns.
- Real-time SUS collection via in-app surveys is becoming more common.
- Integration with platforms like Hotjar, Mixpanel, and UserTesting enhances data depth.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 70 is above average, and scores above 80 are considered good to excellent. However, what constitutes a “good” score depends on the context, industry, and competitive landscape.
Can I modify the System Usability Scale questions?
It’s generally not recommended to modify the wording of the SUS questions, as this can affect the validity of the results. However, you can add custom follow-up questions to gather more specific feedback. The core 10 items should remain unchanged to ensure comparability.
How many users do I need to get a reliable SUS score?
Research suggests that as few as 5–8 users can provide a reliable estimate of a system’s usability when using SUS. For more precise benchmarking or comparative studies, 15–20 users are recommended.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for anyone to use. There are no licensing fees or restrictions. You can find the official questionnaire and scoring guide on resources like MeasuringU or the Nielsen Norman Group website.
How does SUS compare to other usability metrics?
SUS is a subjective, self-reported measure of perceived usability. It complements objective metrics like task success rate, time on task, and error rate. While SUS tells you how users feel about a system, performance metrics tell you how well they perform. Using both provides a complete picture.
The System Usability Scale remains one of the most trusted tools in the UX professional’s toolkit. Its simplicity, reliability, and versatility make it indispensable for evaluating how users perceive the ease of use of a system. While it has limitations — particularly in diagnostic depth — its value lies in providing a quick, standardized snapshot of usability. When used correctly and in conjunction with other methods, SUS empowers teams to make informed design decisions, improve user satisfaction, and ultimately build better products. As technology continues to evolve, the system usability scale will likely remain a cornerstone of usability evaluation for years to come.
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