System Usability Scale: 10 Powerful Insights You Must Know

admin4 weeks ago

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Ever wondered how to measure if a product is truly user-friendly? The System Usability Scale (SUS) is the gold standard for evaluating usability—simple, reliable, and widely trusted across industries.

Table of Contents

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 1986 by John Brooke at Digital Equipment Corporation, it has since become one of the most widely used tools in usability testing. Despite its age, the SUS remains remarkably relevant due to its simplicity, flexibility, and strong psychometric properties.

Origins and Development of SUS

The System Usability Scale was created during usability research at Digital Equipment Corporation (DEC) to provide a quick and effective way to evaluate user experience without requiring complex analysis. John Brooke aimed to design a tool that could be administered easily after any usability test, regardless of the technology being tested—be it software, websites, mobile apps, or even medical devices.

Brooke’s goal was not to create an exhaustive usability metric but rather a lightweight, reliable scale that could produce consistent results across different contexts. The original research was published in a little-known technical report, but over time, SUS gained traction through word-of-mouth and academic citations. Today, it’s referenced in thousands of research papers and industry case studies.

Developed in 1986 by John Brooke
Originally intended for internal use at DEC
Published in a technical report titled “SUS: A Quick and Dirty Usability Scale”

Despite its informal nickname (“quick and dirty”), the SUS has proven to be anything but dirty—its reliability and validity have been confirmed through decades of empirical research. It’s been applied in fields ranging from healthcare to aviation, e-commerce to education, making it one of the most versatile tools in human-computer interaction (HCI).

How the SUS Questionnaire Works

The core of the System Usability Scale is a 10-statement questionnaire answered on a 5-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 users complete a task or interaction with a system, they are asked to respond to these statements.

Here are the 10 standard SUS items:

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.I think that I would need the support of a technical person to be able to use this system.I found the various functions in this system were well integrated.I thought there was too much inconsistency in this system.I would imagine that most people would learn to use this system very quickly.I found the system very cumbersome to use.I felt very confident using the system.I needed to learn a lot of things before I could get going with this system.Notice how odd-numbered items are positively worded, while even-numbered ones are negatively worded.This design helps prevent users from mechanically agreeing or disagreeing with all statements.

.After responses are collected, a specific scoring algorithm is applied to calculate the final SUS score..

“The beauty of the SUS lies in its simplicity—it doesn’t tell you *why* something is unusable, but it reliably tells you *if* it is.” — Jakob Nielsen, Nielsen Norman Group

Why the System Usability Scale Is So Widely Used

The enduring popularity of the System Usability Scale can be attributed to several key strengths that make it stand out among usability assessment tools. Unlike more complex usability metrics that require extensive training or specialized software, SUS is accessible, fast, and produces actionable data.

Simplicity and Ease of Administration

One of the biggest advantages of the System Usability Scale is its ease of use. Researchers, designers, and product managers can administer the SUS without needing advanced statistical training. The questionnaire takes less than 5 minutes to complete, making it ideal for integration into usability tests, beta programs, or customer feedback loops.

Because it’s technology-agnostic, the same SUS form can be used to evaluate a mobile banking app, a hospital patient portal, or a smart home device. This universality makes it a favorite among UX professionals who work across multiple platforms.

Additionally, the scoring process is straightforward. Once responses are collected, each item is scored based on whether it’s positively or negatively worded. For odd-numbered items (positive), you subtract 1 from the response value. For even-numbered items (negative), you subtract the response value from 5. Then, you sum all adjusted scores and multiply by 2.5 to get a final score between 0 and 100.

Takes under 5 minutes to complete
No special training required
Works across digital and physical systems

This simplicity doesn’t come at the cost of reliability. Studies have shown that SUS has high internal consistency (Cronbach’s alpha typically above 0.9), meaning the items in the questionnaire are measuring the same underlying construct—usability.

Proven Reliability and Validity

Despite its brevity, the System Usability Scale has strong psychometric properties. Numerous studies have confirmed its reliability and validity across different languages, cultures, and domains. A meta-analysis by Bangor, Kortum, and Miller (2008) validated the SUS across hundreds of studies and established normative data for interpreting scores.

The researchers found that SUS scores follow a roughly normal distribution and proposed a grading scale:

Below 50: Poor
50–60: Acceptable
60–70: Good
70–80: Excellent
80–100: Best Imaginable

This benchmarking allows teams to compare their product’s usability not just against previous versions, but against industry standards. For example, a SUS score of 68 might seem decent, but knowing that the average for software applications is around 68 means there’s room for improvement.

Moreover, SUS correlates well with other usability metrics such as task success rate, time-on-task, and user satisfaction, making it a holistic indicator of user experience. You can read more about this validation in the comprehensive review by Bangor, Kortum, and Miller.

How to Calculate the System Usability Scale Score

Calculating the SUS score is a systematic process that ensures consistency and accuracy. While the math is simple, understanding the logic behind the scoring helps ensure correct interpretation.

Step-by-Step Scoring Methodology

To calculate the SUS score, follow these steps:

For each odd-numbered item (1, 3, 5, 7, 9), subtract 1 from the user’s response (e.g., if the user answered “4,” compute 4 – 1 = 3).
For each even-numbered item (2, 4, 6, 8, 10), subtract the user’s response from 5 (e.g., if the user answered “2,” compute 5 – 2 = 3).
Sum all the adjusted values from steps 1 and 2.
Multiply the total sum by 2.5 to convert it to a 0–100 scale.

Let’s walk through an example. Suppose a user responds to the 10 SUS items with the following answers: [4, 1, 5, 1, 5, 1, 5, 1, 5, 1].

Item 1 (odd): 4 – 1 = 3
Item 2 (even): 5 – 1 = 4
Item 3 (odd): 5 – 1 = 4
Item 4 (even): 5 – 1 = 4
Item 5 (odd): 5 – 1 = 4
Item 6 (even): 5 – 1 = 4
Item 7 (odd): 5 – 1 = 4
Item 8 (even): 5 – 1 = 4
Item 9 (odd): 5 – 1 = 4
Item 10 (even): 5 – 1 = 4

Sum = 3 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 = 41

Final SUS Score = 41 × 2.5 = 102.5 → capped at 100 (maximum possible score)

This hypothetical user gave mostly high agreement to positive statements and strong disagreement to negative ones, resulting in a near-perfect score.

Common Scoring Mistakes to Avoid

While the SUS scoring formula is straightforward, errors can occur—especially when processing multiple responses manually. Here are common pitfalls:

Forgetting to reverse-score even-numbered items: This is the most frequent mistake. If you treat all items the same, your results will be invalid.
Not multiplying by 2.5: Without this step, the score remains on a 0–40 scale instead of 0–100.
Using averages before transformation: Never average raw responses before applying the formula. Always calculate individual SUS scores first, then average them across users.
Ignoring missing data: If a user skips a question, the SUS cannot be scored reliably. Some researchers impute missing values, but this is not recommended unless absolutely necessary.

To avoid these issues, many teams use automated tools or templates. For instance, the SUS Calculator by MeasuringU allows you to input responses and instantly get the score, reducing human error.

“A single SUS score won’t fix your design, but a trend of improving SUS scores over time means you’re moving in the right direction.” — Dr. James Lewis, IBM Senior Usability Researcher

Interpreting System Usability Scale Results

Getting a number is only half the battle—the real value of the System Usability Scale comes from interpreting what that number means in context.

Understanding the SUS Score Range

The SUS score ranges from 0 to 100, with higher scores indicating better perceived usability. As mentioned earlier, Bangor et al. (2008) established a widely accepted grading scale:

0–50: Poor usability. Users are likely frustrated and may abandon the system.
51–60: Marginal. The system works but has significant usability issues.
61–70: Acceptable. Meets basic usability expectations.
71–80: Good. Users find the system intuitive and efficient.
81–100: Excellent. Represents best-in-class usability.

It’s important to note that the average SUS score across all systems is approximately 68. So, a score of 70 isn’t just “good”—it’s slightly above average. A score of 80 puts you in the top tier of user-friendly products.

For example, Google Search consistently scores above 85, while enterprise software often hovers around 60–65. Knowing these benchmarks helps set realistic goals for your product.

Comparative Analysis and Benchmarking

One of the most powerful uses of the System Usability Scale is comparative testing. You can use SUS to:

Compare two design prototypes (A vs. B)
Measure usability before and after a redesign
Compare your product to competitors
Track usability improvements over time

For instance, if your current app has a SUS score of 62 and a new prototype scores 75, that’s a statistically significant improvement (assuming adequate sample size). Even a 10-point increase can reflect meaningful enhancements in user experience.

system usability scale – System usability scale menjadi aspek penting yang dibahas di sini.

When benchmarking against competitors, you can conduct side-by-side tests where the same users evaluate multiple systems. This within-subjects design increases sensitivity and reduces variability. Just ensure tasks are comparable and order effects are counterbalanced.

Another approach is to use publicly available SUS benchmarks. For example, the Nielsen Norman Group publishes industry averages for various product types, helping you contextualize your results.

Advantages and Limitations of the System Usability Scale

Like any tool, the System Usability Scale has strengths and weaknesses. Understanding both is crucial for using it effectively.

Key Advantages of Using SUS

The SUS offers several compelling benefits that explain its longevity and widespread adoption:

Brevity: Only 10 questions, taking less than 5 minutes to complete.
Reliability: High internal consistency and test-retest reliability.
Flexibility: Applicable to any interactive system, regardless of domain or platform.
Standardization: Enables cross-study and cross-product comparisons.
Free to use: No licensing fees or restrictions—SUS is in the public domain.

Because it’s free and easy to implement, SUS is especially valuable for startups, academic researchers, and small UX teams with limited resources. It democratizes access to high-quality usability measurement.

Known Limitations and Criticisms

Despite its strengths, the System Usability Scale is not without limitations:

Doesn’t diagnose problems: SUS tells you *how usable* a system is, but not *why* it’s unusable.For root cause analysis, you need qualitative methods like interviews or think-aloud protocols.Subjective measure: It captures perceived usability, not objective performance (e.g., error rates, task completion time).Limited granularity: A single score may mask usability issues in specific features or user segments.Cultural bias concerns: Some studies suggest response styles (e.g., acquiescence bias) may affect scores in different cultures.No built-in emotional dimension: SUS focuses on usability, not delight, engagement, or emotional response.To overcome these limitations, many organizations combine SUS with other tools.

.For example, pairing SUS with the User Experience Questionnaire (UXQ) or Net Promoter Score (NPS) provides a more holistic view of user experience..

“SUS is like a thermometer for usability—it tells you if the patient has a fever, but you still need a doctor to find the cause.” — UX Design Collective

Best Practices for Administering the System Usability Scale

To get the most accurate and actionable results from the System Usability Scale, it’s essential to follow best practices in administration and data collection.

When and How to Deploy SUS

The timing and context of SUS administration significantly impact data quality. Best practices include:

Administer after a task-based session: SUS should follow a usability test where users have interacted with the system. Don’t give it as a standalone survey without context.
Use it formatively and summatively: In early design phases (formative), SUS helps identify issues. In final testing (summative), it validates overall usability.
Keep the instructions neutral: Avoid leading questions or biased language when introducing the survey.
Ensure anonymity: Users are more honest when they know their responses are confidential.

For remote testing, tools like Lookback, UserTesting, or Maze can automatically deliver the SUS after a session. In lab settings, it can be handed out on paper or via tablet.

Recommended Sample Size and Statistical Considerations

While SUS can be used with small samples (even n=5), larger samples provide more reliable and generalizable results. Here’s a guideline:

n = 5–10: Useful for quick feedback in early prototypes.
n = 15–20: Provides reasonable confidence for internal reports.
n = 30+: Recommended for publication, benchmarking, or high-stakes decisions.

Because SUS produces interval-level data, you can apply parametric statistics like t-tests or ANOVA to compare groups. However, always check for normality, especially with small samples. Non-parametric tests (e.g., Mann-Whitney U) are safer alternatives when data is skewed.

Confidence intervals are also valuable. A SUS score of 72 ± 8 (95% CI) gives more insight than a point estimate alone.

Alternatives and Complements to the System Usability Scale

While the System Usability Scale is powerful, it’s not the only usability metric available. Depending on your goals, you might consider alternatives or complementary tools.

Combining SUS With Qualitative Methods

The most effective usability evaluations combine quantitative metrics like SUS with qualitative insights. After users complete the SUS, follow up with open-ended questions such as:

What did you find most confusing?
What one change would make this system easier to use?
How would you describe this system to a friend?

This mixed-methods approach gives you both the “what” (from SUS) and the “why” (from interviews). For instance, a low SUS score combined with recurring complaints about navigation can pinpoint specific design flaws.

Tools like Optimal Workshop or Maze integrate SUS with tree testing, first-click testing, and feedback collection, enabling comprehensive UX evaluation.

What is a good System Usability Scale score?

A score above 68 is considered above average, while 70–80 is good, and 80+ is excellent. However, what’s “good” depends on your industry and competitors. Always benchmark against relevant standards.

Can I modify the SUS questionnaire?

While you can rephrase items for clarity, modifying the SUS is not recommended as it may invalidate the scoring and prevent comparison with established benchmarks. If you need a customized tool, consider developing a new questionnaire or using an alternative like UMUX.

Is the System Usability Scale free to use?

Yes, the System Usability Scale is in the public domain and free for any use—commercial or academic. No permission is required, though citing the original authors (Brooke, 1986) is good practice.

How many users do I need for a reliable SUS score?

As few as 5 users can reveal major usability issues, but for reliable benchmarking, aim for 15–30 users. Larger samples reduce margin of error and increase statistical power.

Can SUS be used for non-digital products?

Absolutely. The System Usability Scale has been successfully applied to physical products like medical devices, automotive interfaces, and consumer appliances. Its flexibility is one of its greatest strengths.

The System Usability Scale remains a cornerstone of usability evaluation because it strikes the perfect balance between simplicity and effectiveness. While it doesn’t replace in-depth user research, it provides a fast, reliable, and standardized way to measure perceived usability. By understanding how to administer, score, and interpret SUS correctly—and by combining it with qualitative insights—you can make data-driven decisions that significantly improve user experience. Whether you’re a UX researcher, product manager, or designer, mastering the SUS is a powerful step toward building more usable, user-centered products.