Ever wondered how to measure if a product is truly user-friendly? Enter the System Usability Scale (SUS)—a simple yet powerful tool that turns subjective feelings into reliable data. Let’s dive into why it’s a game-changer.
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 the late 1980s by John Brooke at Digital Equipment Corporation, SUS has become one of the most widely used tools in usability testing across industries—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
The System Usability Scale was first introduced in 1986 as a quick, reliable way to assess usability without requiring complex metrics or extensive user testing. Brooke aimed to create a tool that was both technology-agnostic and easy to administer. Despite its age, SUS remains relevant because of its simplicity, flexibility, and strong psychometric properties.
Unlike other usability scales that are tied to specific platforms or require expert interpretation, SUS can be applied to almost any interactive system. Its enduring popularity stems from its ability to deliver consistent, comparable results with minimal effort. Over the decades, it has been validated across cultures, languages, and domains, making it a gold standard in usability research.
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.
- I found the system unnecessarily complex.
- I thought the system was easy to use.
After users complete the survey, a specific scoring algorithm is applied: odd-numbered items are scored by subtracting 1 from the user response, while even-numbered items are scored by subtracting the user response from 5. These scores are then summed and multiplied by 2.5 to produce a final score between 0 and 100.
“The beauty of the System Usability Scale lies in its simplicity—it’s short, reliable, and gives you a single number that speaks volumes.” — Jakob Nielsen, Nielsen Norman Group
Why the System Usability Scale Matters in UX Design
In the world of user experience (UX) design, intuition isn’t enough. Designers and product teams need objective data to validate assumptions and guide decisions. The System Usability Scale fills this gap by providing a standardized, quantifiable measure of usability that can be tracked over time and compared across products.
Quantifying Subjective User Experience
Usability is inherently subjective—what feels intuitive to one user might confuse another. The System Usability Scale transforms these subjective impressions into a concrete metric. By aggregating responses from multiple users, teams can identify trends, benchmark performance, and detect usability issues that might not surface in observational testing alone.
For example, a product might perform well in task completion rates during a usability test, but if users rate it poorly on SUS, it suggests underlying friction—perhaps in navigation, terminology, or workflow logic. This insight is critical for iterative design improvements.
Benchmarking and Comparative Analysis
One of the most powerful features of the System Usability Scale is its ability to support benchmarking. Because SUS scores are normalized on a 0–100 scale, they allow for direct comparisons between different systems, versions of the same product, or even competitors.
Research by the Nielsen Norman Group indicates that the average SUS score across thousands of studies is around 68. A score above 68 is considered above average, while below 68 is below average. This benchmark provides context: a score of 75 might seem good in isolation, but knowing it’s only slightly above average can motivate further refinement.
Companies often use SUS to compare pre- and post-redesign versions of a product. A significant increase in SUS score after a redesign provides strong evidence that usability has improved, which can be crucial for stakeholder buy-in and ROI justification.
How to Administer the System Usability Scale
Administering the System Usability Scale is straightforward, but doing it correctly ensures reliable and actionable results. Whether you’re conducting a lab study, remote test, or field evaluation, following best practices is essential.
When to Use SUS in Testing
The ideal time to deploy the SUS is immediately after a user completes a set of representative tasks with the system. This ensures that their experience is fresh and contextually grounded. SUS should not be used in isolation—it works best when combined with qualitative methods like think-aloud protocols, interviews, or observational notes.
For instance, in a usability test for a new e-commerce app, participants might be asked to search for a product, add it to the cart, and complete a mock purchase. Right after this sequence, they would complete the SUS questionnaire. This timing captures their overall impression of the experience, not just isolated moments.
Best Practices for Deployment
To get the most out of the System Usability Scale, follow these guidelines:
- Use it consistently: Apply the same version of SUS across all test sessions to ensure comparability.
- Keep it anonymous: Allow users to respond honestly without fear of judgment.
- Pair with qualitative feedback: Always ask follow-up questions like “What did you find most confusing?” to enrich the numerical data.
- Test with at least 12 users: While SUS can be used with small samples, larger groups (12–15 users) provide more stable and reliable scores.
Additionally, ensure that the instructions are clear and neutral. Avoid leading questions or influencing the user’s perception before they complete the survey. Tools like SurveyMonkey or Typeform can help automate distribution and scoring.
Scoring and Interpreting SUS Results
One of the most appealing aspects of the System Usability Scale is its straightforward scoring method. However, understanding how to interpret the results is just as important as calculating them correctly.
The SUS Scoring Formula Explained
Each of the 10 SUS items is scored as follows:
- For odd-numbered items (1, 3, 5, 7, 9): Subtract 1 from the user’s response (e.g., if the user selects 4, the score is 3).
- For even-numbered items (2, 4, 6, 8, 10): Subtract the user’s response from 5 (e.g., if the user selects 2, the score is 3).
These adjusted scores are summed, then multiplied by 2.5 to yield a final score between 0 and 100. For example, if the raw sum is 34, the SUS score is 34 × 2.5 = 85.
While manual calculation is possible, many researchers use online calculators or spreadsheet templates. The MeasuringU website offers a free SUS calculator and detailed tutorials on proper scoring.
Understanding SUS Score Ranges and Benchmarks
Interpreting a SUS score requires context. Here’s a general guideline based on extensive research:
- 90–100: Excellent (top 10% of systems)
- 80–89: Good
- 70–79: Acceptable
- 60–69: Poor
- 50–59: Awful
- Below 50: Unacceptable
A score of 68 is the historical average, so any result above that is considered above average. However, industry and user expectations matter. A banking app with a 75 might still face criticism if users expect seamless, frictionless experiences.
It’s also valuable to look at the distribution of scores. If most users score between 80 and 90, the product is consistently well-received. But if scores range from 40 to 90, it suggests inconsistent usability—some users find it intuitive, while others struggle significantly.
Advantages of the System Usability Scale
The widespread adoption of the System Usability Scale is no accident. Its popularity is rooted in a set of compelling advantages that make it a go-to tool for UX professionals, product managers, and researchers.
Simplicity and Ease of Use
One of the biggest strengths of the System Usability Scale is its simplicity. The questionnaire takes less than 10 minutes to complete, minimizing user burden. This brevity increases response rates and reduces fatigue, especially in longer usability studies.
Moreover, the scoring algorithm, while not immediately obvious, is easy to automate. Once set up, calculating SUS scores becomes a routine part of data analysis. This low overhead makes it accessible even to teams with limited UX resources.
Reliability and Validity Across Contexts
Despite its simplicity, the System Usability Scale is remarkably robust. Numerous studies have confirmed its reliability (consistency of results) and validity (accuracy in measuring what it claims to measure).
Research published in the Journal of Usability Studies has shown that SUS has high internal consistency, with Cronbach’s alpha typically above 0.9, indicating strong reliability. It also correlates well with other usability metrics like task success rates and user satisfaction.
Crucially, SUS performs well across different languages and cultures. Translated versions have been validated in dozens of countries, from Japan to Brazil, maintaining its psychometric integrity. This cross-cultural applicability makes it ideal for global product teams.
Limitations and Criticisms of SUS
While the System Usability Scale is a powerful tool, it’s not without limitations. Understanding its weaknesses helps teams use it more effectively and know when to supplement it with other methods.
Lack of Diagnostic Depth
One of the most common criticisms of the System Usability Scale is that it doesn’t tell you *why* a system is usable or not. A low score signals a problem, but it doesn’t pinpoint the cause. Was it confusing navigation? Poor error messages? Slow performance?
To address this, SUS should always be paired with qualitative feedback. Follow-up questions like “What was the most frustrating part of using the system?” or “Where did you feel lost?” can uncover the root causes behind the score.
Some researchers have proposed expanded versions of SUS, such as the SUS+ or the Usability Metric for User Experience (UMUX), which add diagnostic questions while preserving the original SUS structure.
Sensitivity to Task Design and Context
The SUS score can be influenced by factors outside the system’s actual usability. For example, if users are given poorly designed tasks during testing, they may rate the system lower—even if the interface itself is sound.
Similarly, user expertise plays a role. A system designed for experts might receive low SUS scores from novices, not because it’s poorly designed, but because it assumes prior knowledge. This context dependency means SUS results must be interpreted with care and in conjunction with user demographics and task scenarios.
Practical Applications of the System Usability Scale
The System Usability Scale isn’t just a theoretical tool—it’s used in real-world settings across industries to drive product improvement, support design decisions, and demonstrate value to stakeholders.
Use in Software and Web Development
In software and web development, SUS is often used during beta testing, usability labs, and post-launch evaluations. For example, a SaaS company might run a SUS survey after onboarding new users to assess how intuitive the setup process is.
Development teams use SUS scores to prioritize bug fixes and feature enhancements. A drop in SUS after a new release can trigger a usability review, helping teams catch issues before they impact retention.
Companies like Google and Microsoft have incorporated SUS-like metrics into their internal UX assessment frameworks, demonstrating its value at scale.
Application in Healthcare and Medical Devices
In high-stakes environments like healthcare, usability isn’t just about convenience—it’s a matter of safety. The System Usability Scale is widely used to evaluate medical devices, electronic health records (EHRs), and telehealth platforms.
Regulatory bodies like the FDA encourage usability testing for medical devices, and SUS provides a standardized way to document usability performance. A high SUS score can support regulatory submissions by demonstrating that a device is intuitive and error-resistant.
For example, a study on a new infusion pump interface might use SUS to compare two design variants. The version with the higher SUS score would be more likely to reduce user errors in clinical settings.
Integration in Academic and Industry Research
The System Usability Scale is one of the most cited tools in human-computer interaction (HCI) research. Its presence in academic literature spans decades, with thousands of studies referencing SUS as a primary or secondary metric.
Researchers appreciate its ease of use, statistical robustness, and compatibility with other methods. It’s frequently used in comparative studies, longitudinal evaluations, and cross-cultural research.
Industry researchers also rely on SUS for competitive analysis. By testing rival products with the same user group and measuring SUS scores, companies can position their offerings more effectively in the market.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 70 is generally seen as acceptable, while scores above 80 are good, and above 90 are excellent. However, what’s “good” depends on the context, industry, and user expectations.
Can the System Usability Scale be used for mobile apps?
Yes, absolutely. The System Usability Scale is platform-agnostic and is widely used for mobile apps, websites, desktop software, and even physical devices with digital interfaces. Its flexibility makes it ideal for evaluating mobile user experiences.
Is the SUS questionnaire 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. However, proper attribution to John Brooke is recommended in academic or professional publications.
How many users do I need for a reliable SUS score?
While SUS can be used with as few as 5 users, a sample size of 12–15 is recommended for more stable and reliable results. Larger samples reduce the margin of error and increase confidence in the findings.
Are there alternatives to the System Usability Scale?
Yes, alternatives include the UMUX (Usability Metric for User Experience), UMUX-Lite, and the SUPR-Q (Standardized User Experience Percentile Rank Questionnaire). These tools offer similar functionality, with some providing additional dimensions like trust or loyalty.
The System Usability Scale remains a cornerstone of usability evaluation for good reason—it’s simple, reliable, and universally applicable. From its humble beginnings in a corporate lab to its current status as a global standard, SUS continues to empower teams to build better, more user-friendly products. While it has limitations, its strengths far outweigh them, especially when used as part of a broader UX research strategy. Whether you’re a designer, developer, or researcher, mastering the SUS is a critical step toward creating experiences that truly work for people.
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