Ever wondered how to measure how user-friendly a system really is? Enter the System Usability Scale (SUS)—a simple, reliable tool that’s become the gold standard in usability evaluation. Let’s dive into what makes it so powerful.
What Is the System Usability Scale (SUS)?
The System Usability Scale (SUS) is a widely used, ten-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed by John Brooke in 1986, it has stood the test of time due to its simplicity, reliability, and versatility across different technologies and industries.
Origins and Development of SUS
The System Usability Scale was originally developed during Brooke’s work at Digital Equipment Corporation (DEC) as a quick way to evaluate the usability of computer systems. Unlike more complex usability testing methods, SUS was designed to be lightweight, easy to administer, and capable of producing actionable data with minimal effort.
Brooke’s goal was to create a tool that could be used across different platforms—whether it was a desktop application, a website, or a mobile app—without requiring extensive customization. This universality is one of the key reasons why the System Usability Scale remains relevant over three decades later.
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. For example:
- I think that I would like to use this system frequently. (Positive)
- I found the system unnecessarily complex. (Negative)
- I thought the system was easy to use. (Positive)
After users complete the survey, a specific scoring algorithm is applied to calculate a final SUS score between 0 and 100. This score provides a quantitative measure of perceived usability.
Why SUS Stands Out Among Usability Tools
While many usability assessment tools exist, the System Usability Scale stands out for several reasons:
- Simplicity: Only 10 questions, quick to administer.
- Reliability: Proven consistent results across diverse user groups and systems.
- Flexibility: Applicable to software, hardware, websites, apps, and even voice interfaces.
- Standardization: Allows for benchmarking against industry averages.
“SUS is not the most detailed usability metric, but it’s the most practical.” — Jakob Nielsen, Nielsen Norman Group
How to Administer the System Usability Scale
One of the biggest strengths of the System Usability Scale is its ease of use. You don’t need a PhD in psychology or human-computer interaction to deploy it effectively. However, following best practices ensures accurate and meaningful results.
When to Use SUS in Your Testing Process
The System Usability Scale can be used at various stages of product development:
- Post-Task Testing: After users complete specific tasks during a usability test.
- Post-Prototype Evaluation: After interacting with a clickable prototype.
- Post-Launch Assessment: To evaluate real-world user experience after deployment.
- Comparative Studies: To compare two versions of a product (e.g., A/B testing).
Because SUS measures perceived usability rather than performance metrics (like task success rate or time-on-task), it complements other usability data perfectly.
Step-by-Step Guide to Administering SUS
Here’s how to properly administer the System Usability Scale:
- Recruit Representative Users: Ensure participants reflect your actual user base in terms of experience, age, and technical proficiency.
- Let Users Interact with the System: Give them realistic tasks to perform. This primes them to answer the SUS questions meaningfully.
- Present the SUS Questionnaire: Immediately after interaction, present the 10-item survey. Delay reduces accuracy.
- Collect Responses Anonymously: Encourage honesty by ensuring privacy.
- Calculate the Score: Use the standard SUS scoring formula (explained below).
For best results, aim for at least 8–12 participants. While SUS works with small samples, larger groups increase statistical confidence.
Common Pitfalls to Avoid
Despite its simplicity, mistakes in administering the System Usability Scale can skew results:
- Administering Too Early: If users haven’t interacted enough with the system, their responses will be speculative.
- Using Non-Representative Users: Testing with tech-savvy insiders won’t reflect real-world usability.
- Modifying the Wording: Even slight changes to SUS questions can invalidate comparisons with benchmarks.
- Ignoring Context: A high SUS score doesn’t explain why users felt a system was usable—always pair SUS with qualitative feedback.
Understanding the System Usability Scale Scoring Method
The scoring mechanism of the System Usability Scale is straightforward but requires attention to detail. Each response contributes to a final score between 0 and 100, where higher scores indicate better perceived usability.
The SUS Scoring Formula Explained
Here’s how to calculate the SUS score manually:
- For odd-numbered items (1, 3, 5, 7, 9), subtract 1 from the user’s response:
score = response - 1 - For even-numbered items (2, 4, 6, 8, 10), subtract the user’s response from 5:
score = 5 - response - Sum all the converted scores.
- Multiply the total by 2.5 to normalize the score to a 0–100 scale.
Example: If a user’s adjusted sum is 40, the final SUS score is 40 × 2.5 = 100.
This formula ensures that the lowest possible score is 0 and the highest is 100, even though raw responses range from 10 to 50.
Interpreting SUS Scores: What Do They Mean?
Raw SUS scores are useful, but interpretation is key. Here’s a widely accepted grading scale:
- 90–100: Excellent (Best in class)
- 80–89: Good
- 70–79: Acceptable (Above average)
- 60–69: Poor
- 50–59: Awful
- Below 50: Unacceptable
According to research by Sauro and Lewis (2006), the average SUS score across thousands of studies is around 68. So, a score above 70 is generally considered above average.
Benchmarking with SUS: Comparing Across Products
One of the most powerful uses of the System Usability Scale is benchmarking. Because SUS is standardized, you can compare your product’s score to industry averages or competitors.
For example:
- An e-commerce platform scoring 85 is likely more user-friendly than one scoring 65.
- A mobile banking app with a SUS of 72 is performing better than the average financial app (which often scores around 65–70).
- Internal enterprise software frequently scores below 60, indicating significant usability challenges.
This comparative power makes the System Usability Scale an essential tool for product managers and UX designers aiming to improve user satisfaction.
Advantages of Using the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. It offers several compelling advantages that make it a go-to tool in both academic and industrial settings.
Speed and Efficiency in Data Collection
One of the biggest advantages of the System Usability Scale is how quickly it can be administered. Most users complete the 10-question survey in under 5 minutes. This makes it ideal for:
- Lean UX teams with limited time.
- Agile development cycles requiring rapid feedback.
- Large-scale studies where time per participant is constrained.
Unlike in-depth interviews or cognitive walkthroughs, SUS provides a quantitative snapshot of usability with minimal overhead.
High Reliability and Validity
Despite its brevity, the System Usability Scale has demonstrated strong psychometric properties:
- Internal Consistency: Cronbach’s alpha typically exceeds 0.9, indicating high reliability.
- Test-Retest Reliability: Users tend to give similar scores when retested under the same conditions.
- Construct Validity: SUS correlates well with other usability metrics like task success and user satisfaction.
These qualities mean that SUS isn’t just a quick fix—it’s a scientifically sound instrument.
Cross-Platform Applicability
Whether you’re testing a VR headset, a smartwatch app, or a government website, the System Usability Scale works. Its language is generic enough to apply across domains without losing meaning.
For instance:
- Healthcare apps use SUS to evaluate patient portals.
- Automotive companies assess infotainment systems with SUS.
- E-learning platforms measure course interface usability using SUS.
This adaptability is rare among usability tools and contributes to its widespread adoption.
Limitations and Criticisms of the System Usability Scale
No tool is perfect, and the System Usability Scale has its share of limitations. Understanding these helps users apply it more effectively and interpret results with appropriate caution.
Lack of Diagnostic Detail
While SUS tells you how usable a system is, it doesn’t explain why. A low score indicates problems, but not where they lie.
For example:
- A SUS score of 55 could stem from poor navigation, confusing labels, slow performance, or any combination of issues.
- Without follow-up questions or observational data, teams may waste time fixing the wrong things.
That’s why SUS should always be paired with qualitative methods like think-aloud protocols or post-test interviews.
Sensitivity to User Expectations and Context
SUS measures perceived usability, which can be influenced by factors outside the system’s control:
- Brand Perception: Users may rate a well-known brand higher, even if the interface is flawed.
- Task Difficulty: If tasks are inherently complex (e.g., tax filing), users may rate usability lower, even if the system is well-designed.
- Emotional State: Frustration from external factors can bleed into SUS responses.
These contextual influences mean that SUS scores should be interpreted alongside other data points.
Language and Cultural Biases
Although the System Usability Scale has been translated into dozens of languages, nuances in translation can affect responses. For example:
- The phrase “I found the system unnecessarily complex” may carry different connotations in collectivist vs. individualist cultures.
- Agreement scales vary in interpretation across cultures—some users avoid extreme responses, while others embrace them.
Researchers must be cautious when comparing SUS scores across different linguistic or cultural groups.
Best Practices for Maximizing the Value of SUS
To get the most out of the System Usability Scale, it’s not enough to just administer the survey. Strategic implementation and integration with other methods amplify its value.
Combine SUS with Qualitative Feedback
The most effective usability evaluations combine quantitative and qualitative data. After users complete the SUS, ask open-ended questions like:
- What did you find most confusing?
- What one change would make this system easier to use?
- Was there any point where you felt stuck?
This hybrid approach gives you both the “what” (from SUS) and the “why” (from interviews), enabling targeted improvements.
Use SUS for Iterative Design Improvements
Don’t treat SUS as a one-time evaluation. Instead, use it iteratively:
- Measure SUS after each design sprint.
- Track score trends over time to assess progress.
- Set usability goals (e.g., “Increase SUS from 65 to 75 in 3 months”).
For example, a fintech startup might run SUS after every prototype update, using the data to prioritize UI refinements before launch.
Automate SUS Collection and Analysis
Manual scoring is error-prone and time-consuming. To scale SUS usage, consider automation:
- Use online survey tools like SurveyMonkey or Typeform with built-in SUS templates.
- Leverage UX platforms like UsabilityHub or UserZoom that auto-calculate SUS scores.
- Integrate SUS into your CI/CD pipeline for continuous usability monitoring.
Automation reduces friction and makes SUS a sustainable part of your UX workflow.
Real-World Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used daily by companies and researchers worldwide to improve real products.
SUS in Healthcare Technology
In healthcare, usability can be a matter of life and death. SUS is used to evaluate:
- Patient-facing apps for medication management.
- EHR (Electronic Health Record) systems used by doctors.
- Telemedicine platforms.
For example, a study published in the Journal of Medical Internet Research used SUS to compare two diabetes management apps, helping clinicians recommend the more user-friendly option.
SUS in Enterprise Software
Internal tools often suffer from poor usability, leading to low adoption and training costs. Companies use SUS to:
- Evaluate CRM, ERP, and HR systems.
- Compare off-the-shelf vs. custom-built solutions.
- Assess the impact of UI redesigns.
A global bank used SUS to evaluate a new internal reporting tool. The initial SUS score was 58. After iterative improvements, it rose to 79—correlating with a 40% increase in employee adoption.
SUS in Consumer Apps and Websites
From social media to e-commerce, consumer-facing digital products rely on SUS to stay competitive. Examples include:
- Testing checkout flows on retail sites.
- Evaluating onboarding experiences in mobile apps.
- Comparing redesigns of news websites.
A streaming service used SUS to compare two navigation layouts. Version A scored 62; Version B scored 81. Based on this, they launched Version B, which later showed higher engagement in A/B tests.
Future of the System Usability Scale: Evolution and Alternatives
While the System Usability Scale remains dominant, new tools and methodologies are emerging. Understanding these helps contextualize SUS’s role in modern UX practice.
Modern Alternatives to SUS
Several newer scales aim to address SUS’s limitations:
- UMUX (Usability Metric for User Experience): A 4-item scale based on ISO 9241-11, designed to be even shorter than SUS.
- UMUX-Lite: A 2-item version that correlates strongly with SUS but is faster to administer.
- Nielsen Norman Group’s SUPR-Q: Measures usability, credibility, loyalty, and appearance in one survey.
While these tools offer advantages in specific contexts, none have displaced SUS as the de facto standard.
Can SUS Adapt to Emerging Technologies?
As technology evolves, so must usability metrics. Researchers are exploring how SUS applies to:
- Voice Interfaces: Can users rate “ease of use” for a voice assistant?
- AR/VR Systems: Does “complexity” mean the same in immersive environments?
- AI-Powered Tools: How do users perceive usability when behavior is unpredictable?
Preliminary studies suggest SUS remains applicable, though supplementary questions may be needed to capture domain-specific issues.
The Enduring Legacy of SUS
Despite the rise of new tools, the System Usability Scale remains a cornerstone of usability evaluation. Its simplicity, reliability, and broad acceptance ensure it will remain relevant for years to come.
As long as digital products need to be evaluated for user-friendliness, the System Usability Scale will have a place in the toolkit of every UX professional.
What is a good System Usability Scale score?
A score above 70 is generally considered acceptable, with 80+ being good and 90+ excellent. The average SUS score across industries is around 68, so anything above that is above average.
Can I modify the SUS questionnaire?
It’s strongly advised not to modify the wording or order of SUS items, as this can invalidate comparisons with established benchmarks. If customization is necessary, consider using alternative scales like UMUX-Lite.
How many users do I need for a reliable SUS score?
As few as 5–8 users can provide a reliable estimate of usability, thanks to SUS’s high reliability. However, for greater statistical confidence, aim for 12–15 users, especially in comparative studies.
Is the System Usability Scale free to use?
Yes, the System Usability Scale is in the public domain and free for both commercial and academic use. No permission is required, though proper citation of Brooke (1986) is recommended.
How does SUS compare to Net Promoter Score (NPS)?
While both are survey-based metrics, SUS measures perceived usability, whereas NPS measures customer loyalty and likelihood to recommend. They assess different constructs but can be used together for a fuller picture of user experience.
The System Usability Scale is more than just a survey—it’s a proven, powerful tool for understanding how users perceive the ease of use of a system. From its simple 10-item structure to its robust scoring and wide applicability, SUS offers unmatched value in usability evaluation. While it has limitations—particularly in diagnostic depth—its strengths in speed, reliability, and benchmarking make it indispensable. By combining SUS with qualitative insights and using it iteratively, teams can drive meaningful improvements in user experience across any digital product. As technology evolves, SUS continues to adapt, maintaining its status as the gold standard in usability measurement.
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