Ever wondered how to measure if your product is user-friendly? The System Usability Scale (SUS) is your go-to tool. Let’s break it down in a practical, no-nonsense way.
Understanding the System Usability Scale (SUS)
The System Usability Scale, or SUS, is a simple, ten-item scale giving a global view of subjective assessments of usability. It was developed by John Brooke in 1986 and remains one of the most reliable tools for measuring usability.
What is SUS?
The SUS is a quick and dirty, yet reliable tool for measuring the usability of a variety of products and services, including hardware, software, mobile devices, websites, and apps.
- Consists of 10 statements rated on a 5-point Likert scale
- Measures effectiveness, efficiency, and satisfaction
- Yields a single score from 0 to 100
“SUS is technology agnostic and can be used for a wide range of interface types.” – John Brooke
Why SUS Still Matters
Despite being developed decades ago, SUS remains relevant due to its simplicity, robustness, and adaptability. It serves as a benchmark for user experience professionals globally.
- Validated across multiple domains
- Provides a reliable benchmark
- Easy to administer and interpret
How the System Usability Scale Works
Understanding how SUS is administered and calculated is crucial to using it effectively in usability testing.
Structure of the Questionnaire
The SUS consists of 10 statements alternating between positive and negative phrasing. Users rate each on a scale from 1 (Strongly Disagree) to 5 (Strongly Agree).
- Odd-numbered items are positive
- Even-numbered items are negative
- Designed to reduce bias
Scoring the SUS
Each item contributes to the overall score. The process involves converting responses to a 0–4 scale, summing them, and multiplying by 2.5 to get a score out of 100.
- No partial scores; only the final SUS score matters
- Scores above 68 are considered above average
- Scores below 50 indicate usability issues
Applications of the System Usability Scale
The SUS is used across industries to evaluate systems from websites to medical devices.
Software and Web Applications
It’s widely used in software development to gauge the usability of interfaces during the design and testing phases.
- Helps in agile UX testing
- Supports iterative design improvements
- Guides product development priorities
Healthcare and Medical Devices
In healthcare, usability is critical. SUS helps ensure that medical devices are safe and easy to use.
- Used in FDA submissions
- Improves patient safety
- Enhances clinician workflow
Interpreting SUS Scores
While the SUS score is on a 0–100 scale, it’s not a percentage. Interpretation involves understanding benchmarks and user expectations.
Benchmarking SUS Scores
A score of 68 is considered average. Scores above 80 are excellent, while those below 50 suggest major usability problems.
- 68 = Average
- 80+ = Excellent usability
- 50 or less = Poor usability
Comparative Analysis
SUS allows comparison across products or versions, making it ideal for A/B testing and iterative improvements.
- Compare designs or prototypes
- Track usability improvements over time
- Identify user pain points
Advantages of Using SUS
The System Usability Scale is popular for good reasons. It’s quick, cost-effective, and provides actionable insights.
Efficiency and Simplicity
SUS can be administered in minutes and interpreted with minimal training.
- Low learning curve
- Works with small sample sizes
- Minimal resources required
Versatility Across Domains
From mobile apps to enterprise systems, SUS adapts to various contexts.
- Applicable to any digital product
- Cross-platform usability testing
- Supports remote and in-person studies
Limitations and Criticisms
Despite its strengths, SUS isn’t perfect. Understanding its limitations helps in designing better usability evaluations.
Lack of Diagnostic Capability
SUS tells you there’s a problem but not what the problem is.
- Doesn’t identify specific issues
- Needs to be supplemented with qualitative methods
- Best used as a first step in usability evaluation
Misinterpretation of Scores
Some practitioners treat SUS scores as percentages or grades, which can be misleading.
- Scores are not absolute
- Context matters in interpretation
- Requires experience to analyze effectively
Best Practices for Implementing SUS
To get the most out of SUS, follow these best practices during administration and analysis.
Timing and Frequency
Administer SUS after a task or session to capture immediate user impressions.
- Use post-task or post-test
- Repeat across multiple sessions
- Track changes over time
Combining with Other Methods
SUS works best when combined with other UX tools like interviews, task analysis, and eye tracking.
- Use in mixed-method studies
- Combine with Net Promoter Score (NPS)
- Pair with qualitative feedback
Advanced Topics in SUS
For UX professionals, understanding the nuances of SUS can unlock deeper insights.
Statistical Analysis
Advanced users apply statistical techniques to analyze SUS data across user segments and time periods.
- Use ANOVA for group comparisons
- Track trends over time
- Segment by demographics
Customizing the SUS
Some adapt the SUS to specific contexts, but care must be taken not to invalidate the scale.
- Maintain the 10-item structure
- Avoid changing wording significantly
- Consider the SUS Alternative (SUS-A) for accessibility
What is the System Usability Scale used for?
It’s used to measure the perceived usability of a product or system quickly and reliably.
How is the SUS score calculated?
Each item is scored on a 0–4 scale, summed, and multiplied by 2.5 to get a score out of 100.
What is a good SUS score?
A score above 68 is considered good, while 80 or more is excellent.
Can SUS be used for mobile apps?
Yes, it’s widely used in mobile app usability testing.
Is SUS enough to evaluate usability?
No, it should be combined with other methods for a complete picture.
Whether you’re designing a new app or improving an existing system, the System Usability Scale offers a fast, reliable way to measure usability. While it’s not a silver bullet, when used wisely, SUS can be a game-changer for user experience professionals.
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