If you’ve ever wondered how to measure the usability of a product accurately and efficiently, the System Usability Scale (SUS) is your golden ticket. Simple, reliable, and widely trusted, this tool has become a cornerstone in UX research across industries.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to evaluate the perceived usability of a system, product, or service. Developed in 1986 by John Brooke at Digital Equipment Corporation, it has since become one of the most widely used tools for measuring usability across digital and non-digital platforms.
Origins and Development of the SUS
The SUS was created during a time when usability testing was becoming increasingly important in human-computer interaction (HCI). John Brooke needed a lightweight, yet effective method to assess user experience without requiring extensive resources or time. The result was a concise 10-question survey that could be administered quickly and yield reliable results.
- Originally developed for internal use in usability labs.
- Published in 1986 but gained widespread popularity in the 2000s.
- Designed to be technology-agnostic—usable for software, websites, apps, and even physical devices.
“The beauty of the SUS lies in its simplicity and robustness. It doesn’t tell you *why* something is unusable, but it tells you *that* it is.” — Jakob Nielsen, UX Pioneer
Structure of the SUS Questionnaire
The SUS consists of 10 statements, each rated on a 5-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The odd-numbered items are positively worded, while the even-numbered ones are negatively worded, helping to reduce response bias.
- Sample Item 1: “I think that I would like to use this system frequently.”
- Sample Item 2: “I found the system unnecessarily complex.”
- Item 5: “I found the various functions in this system were well integrated.”
After collecting responses, a specific scoring algorithm is applied: for odd-numbered items, subtract 1 from the user response; for even-numbered items, subtract the user response from 5. Sum these values, multiply by 2.5, and you get a final score between 0 and 100.
Why the SUS Is Not a Diagnostic Tool
One common misconception about the System Usability Scale is that it can pinpoint specific usability problems. However, it’s important to understand that SUS measures *perceived usability*, not the root causes of poor usability.
- It provides a high-level snapshot of user satisfaction.
- It cannot replace qualitative methods like user interviews or usability testing.
- Best used in conjunction with other UX research tools for deeper insights.
For example, if a mobile banking app scores 50 on the SUS, it signals moderate usability issues, but further investigation—such as heatmaps or session recordings—is needed to identify navigation bottlenecks or confusing interfaces.
How to Administer the System Usability Scale
Administering the SUS correctly is crucial for obtaining valid and reliable data. While the questionnaire is short, the context in which it’s given can significantly influence results.
Best Practices for Survey Distribution
To ensure accurate results, the SUS should be administered immediately after a user completes a set of representative tasks with the system. This timing ensures that their experience is fresh and their feedback is contextually grounded.
- Use post-task or post-session surveys to capture real-time perceptions.
- Avoid administering SUS days after interaction, as recall bias increases.
- Ensure anonymity to encourage honest responses.
Online tools like Usability.gov offer templates and best practices for deploying SUS effectively in both lab and remote settings.
Choosing the Right Sample Size
One of the advantages of the System Usability Scale is that it requires a relatively small sample size to produce meaningful results. Research shows that even 5–8 users can reveal major usability issues, and SUS scores stabilize with around 15–20 participants.
- For formative testing (early design phases), 8–12 users may suffice.
- For summative testing (final product evaluation), aim for 15–30 users for higher statistical confidence.
- Always report your sample size when sharing SUS results for transparency.
According to Sauro and Lewis (2016), the standard error of the mean SUS score decreases significantly as sample size increases, making larger samples ideal for benchmarking.
Common Pitfalls in Administration
Despite its simplicity, the SUS can be misused. One frequent error is altering the wording of the questions, which compromises the validity of the score. The original phrasing must be preserved to maintain comparability across studies.
- Do not rephrase questions to fit specific jargon.
- Avoid adding or removing items—stick to the 10-item format.
- Don’t mix SUS with other survey questions that might bias responses.
Another pitfall is using SUS in isolation. While it gives a great overall score, pairing it with open-ended questions like “What did you find most confusing?” adds qualitative depth.
Scoring and Interpreting the System Usability Scale
Understanding how to calculate and interpret the SUS score is essential for deriving actionable insights. The scoring process is straightforward but must be followed precisely to ensure accuracy.
Step-by-Step Scoring Process
The SUS scoring algorithm transforms raw responses into a single number between 0 and 100. Here’s how it works:
- For each odd-numbered item (1, 3, 5, 7, 9), subtract 1 from the response (so a ‘4’ becomes ‘3’).
- For each even-numbered item (2, 4, 6, 8, 10), subtract the response from 5 (so a ‘2’ becomes ‘3’).
- Sum all the adjusted values.
- Multiply the total by 2.5 to convert it to a 0–100 scale.
For example, if the sum of adjusted scores is 36, multiplying by 2.5 gives a SUS score of 90—indicating excellent usability.
Interpreting the SUS Score: Benchmarks and Percentiles
A SUS score alone doesn’t tell the full story—context matters. However, researchers have established benchmarks to help interpret scores:
- Below 50: Poor usability (significant issues)
- 50–60: Marginal usability
- 60–70: Acceptable usability
- 70–80: Good usability
- 80–100: Excellent usability
According to data compiled by Jeff Sauro, the average SUS score across thousands of studies is approximately 68. A score above 70 places a product in the top 50% of usability performance.
You can also convert SUS scores into percentile ranks using normative data. For instance, a score of 80 corresponds to the 80th percentile, meaning the system is more usable than 80% of other systems tested.
Using SUS for Comparative Analysis
One of the most powerful applications of the System Usability Scale is comparing different versions of a product (e.g., before and after a redesign) or competing products in the market.
- Conduct SUS tests on both versions under the same conditions.
- Calculate mean scores and confidence intervals for statistical comparison.
- A difference of 2.6 points is statistically significant at the 95% confidence level with 15 users per group.
For example, if Version A scores 65 and Version B scores 78 after a UX overhaul, the 13-point improvement suggests a meaningful enhancement in perceived usability.
Advantages of the System Usability Scale
The enduring popularity of the System Usability Scale is no accident. Its strengths make it a go-to tool for UX professionals, product managers, and researchers worldwide.
Reliability and Validity
Despite its brevity, the SUS has demonstrated strong psychometric properties. Numerous studies have confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
- Cronbach’s alpha, a measure of internal consistency, typically exceeds 0.9 for SUS—indicating high reliability.
- It correlates well with other usability metrics like task success rate and time-on-task.
- It’s been validated across cultures, languages, and domains.
A study by Bangor, Kortum, and Miller (2008) confirmed that SUS produces consistent results even when translated into multiple languages, making it ideal for global product testing.
Speed and Simplicity
In fast-paced development environments, time is a critical factor. The SUS can be completed in under 5 minutes, making it minimally intrusive for users while still delivering valuable data.
- Easy to administer during usability testing sessions.
- Simple scoring allows for rapid analysis.
- Does not require advanced statistical knowledge to interpret.
This efficiency makes the System Usability Scale particularly useful in agile environments where quick feedback loops are essential.
Cost-Effectiveness
Unlike complex usability labs or eye-tracking studies, the SUS requires no special equipment. It can be deployed via email, embedded in apps, or used in person with minimal cost.
- No licensing fees—SUS is free to use.
- Can be integrated into existing user research workflows.
- Reduces the need for large-scale studies due to small sample requirements.
Organizations of all sizes—from startups to Fortune 500 companies—leverage SUS because it delivers high ROI on user research investments.
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is powerful, it’s not without limitations. Understanding these weaknesses helps researchers use it more effectively and avoid misinterpretation.
Lack of Diagnostic Detail
The SUS provides a global usability score but does not explain *why* users feel a certain way. A low score indicates problems, but not where they lie.
- Does not identify specific pain points (e.g., confusing menu, slow load time).
- Cannot replace observational methods like think-aloud protocols.
- May mask usability issues if users are generally satisfied despite flaws.
For deeper insights, always pair SUS with qualitative feedback or behavioral analytics.
Sensitivity to Context and Task Design
The SUS score can be influenced by how the preceding tasks are designed. If users struggle due to poorly designed tasks rather than the system itself, the SUS score may unfairly suffer.
- Ensure tasks are realistic and representative of actual usage.
- Avoid overly complex or ambiguous instructions.
- Test the task flow with a pilot group before full deployment.
For example, asking a user to complete a multi-step purchase process in a single session may lead to fatigue, skewing SUS scores downward even if the interface is sound.
Subjectivity and Cultural Bias
Because SUS relies on self-reported perceptions, it’s subject to individual biases. Cultural differences in response tendencies (e.g., some cultures tend to agree more) can also affect scores.
- Some users may rate systems higher due to brand loyalty.
- Response styles vary across demographics and regions.
- Translation nuances can alter question interpretation.
To mitigate this, use SUS as part of a mixed-methods approach and consider demographic segmentation when analyzing results.
Practical Applications of the System Usability Scale
The versatility of the System Usability Scale makes it applicable across a wide range of industries and use cases. From healthcare to e-commerce, SUS helps organizations make data-driven design decisions.
Use in Software and App Development
In software development, SUS is frequently used during usability testing phases to evaluate prototypes and final products.
- Compare usability of different UI layouts.
- Assess impact of new features on user experience.
- Validate design changes post-redesign.
For instance, a fintech startup might use SUS to compare two onboarding flows, choosing the one with the higher SUS score for launch.
Application in Healthcare Systems
In healthcare, where usability can directly impact patient safety, the System Usability Scale is used to evaluate electronic health record (EHR) systems, medical devices, and telehealth platforms.
- Ensure clinicians can navigate systems efficiently under stress.
- Reduce cognitive load in high-stakes environments.
- Meet regulatory requirements for human factors engineering.
A study published in the Journal of Biomedical Informatics found that EHR systems with higher SUS scores correlated with fewer medication errors and faster documentation times.
Role in Academic and UX Research
The System Usability Scale is a staple in academic research due to its reliability and ease of use. It’s frequently cited in peer-reviewed journals and used in comparative studies.
- Supports hypothesis testing in human-computer interaction (HCI).
- Used in longitudinal studies to track usability over time.
- Facilitates meta-analyses across multiple systems.
Researchers at universities often combine SUS with NASA-TLX (task load index) to assess both usability and mental workload.
Alternatives and Complements to the System Usability Scale
While the System Usability Scale is highly effective, it’s not the only tool available. Depending on your goals, you might consider alternatives or complementary methods.
Usability Questionnaires Similar to SUS
Several other standardized questionnaires exist that serve similar purposes:
- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO 9241, highly correlated with SUS.
- UMUX-Lite: A 2-item version of UMUX, ideal for quick mobile surveys.
- Nielsen’s QUIS (Questionnaire for User Interaction Satisfaction): More detailed but longer to administer.
According to Sauro (2011), UMUX correlates at 0.96 with SUS, making it a viable alternative when brevity is paramount.
Behavioral and Performance Metrics
To go beyond self-reported data, researchers often combine SUS with objective performance metrics:
- Task Success Rate: Percentage of users who complete a task.
- Time on Task: How long it takes to complete a task.
- Error Rate: Number of mistakes made during interaction.
For example, a website redesign might show improved SUS scores *and* reduced time-on-task, providing convergent evidence of enhanced usability.
Qualitative Methods to Enhance SUS
Adding qualitative methods enriches the insights gained from the System Usability Scale:
- Think-Aloud Protocols: Users verbalize thoughts while interacting.
- User Interviews: Post-test interviews to explore feedback in depth.
- Card Sorting: Helps understand information architecture issues.
These methods help answer the “why” behind the SUS score, turning numbers into actionable design recommendations.
Future of the System Usability Scale in UX Research
As technology evolves, so too does the role of the System Usability Scale. While some question whether a 1986 tool remains relevant in the age of AI and voice interfaces, SUS continues to adapt and thrive.
Adaptation to Emerging Technologies
The System Usability Scale is being successfully applied to new domains such as voice assistants, augmented reality (AR), and wearable devices.
- Used to evaluate smart speakers like Amazon Alexa and Google Home.
- Applied in VR training simulations to assess interface intuitiveness.
- Helps optimize wearables for quick, glanceable interactions.
Researchers are exploring minor adaptations—such as rephrasing “system” to “device” or “interface”—while preserving the core structure to maintain comparability.
Integration with AI and Analytics Platforms
Modern UX tools are integrating SUS with AI-powered analytics to provide real-time usability dashboards.
- Automated SUS scoring in platforms like UserTesting and Lookback.
- AI-driven sentiment analysis of open-ended SUS follow-ups.
- Predictive modeling of SUS scores based on user behavior patterns.
These integrations allow teams to monitor usability continuously, not just in isolated testing sessions.
Potential Evolution of SUS
While the original SUS remains unchanged, researchers are exploring ways to enhance it without sacrificing its simplicity.
- Adding a single open-ended question: “What was the one thing you liked most?” and “What was the one thing you found most frustrating?”
- Developing domain-specific variants (e.g., SUS-HC for healthcare).
- Exploring adaptive SUS versions that adjust based on user responses.
However, any changes must be carefully validated to ensure they don’t undermine the metric’s reliability.
What is the System Usability Scale?
The System Usability Scale (SUS) is a 10-item questionnaire used to measure the perceived usability of a system, product, or service. It produces a single score from 0 to 100, with higher scores indicating better usability.
How do you calculate a SUS score?
To calculate a SUS score: adjust each response (subtract 1 for odd items, subtract from 5 for even items), sum the adjusted values, then multiply by 2.5. The result is a score between 0 and 100.
What is a good SUS score?
A SUS score above 68 is considered above average. Scores above 70 are good, and those above 80 are excellent. A score below 50 indicates significant usability problems.
Can I modify the SUS questionnaire?
No, the original wording should not be changed to maintain validity and comparability. However, you can add follow-up qualitative questions to gain deeper insights.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for anyone to use without licensing fees.
The System Usability Scale remains one of the most trusted, efficient, and versatile tools in the UX researcher’s toolkit. Its ability to deliver reliable usability metrics with minimal effort has cemented its place in both academic and industry settings. While it has limitations—particularly its lack of diagnostic detail—its strengths far outweigh its weaknesses when used appropriately. By combining SUS with qualitative methods and performance metrics, teams can gain a holistic understanding of user experience. As technology advances, the SUS continues to evolve, proving that even a simple 10-question survey can have a lasting impact on how we design human-centered systems.
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