The most common answer to "what's the difference between UI and UX" is: UI is appearance, UX is comfort. This answer is correct, but it's difficult to use for making any decisions. Two people can debate for an hour about "comfort" without anyone winning. The three tools below turn comfort into something that can be measured.
Standard definition: usability has three metrics
ISO 9241-11:2018 defines usability as the extent to which a specific user can use a system, product, or service to achieve specific goals, with effectiveness, efficiency, and satisfaction, in a specific context of use. These three metrics can be translated into numbers.
| Metric | Meaning according to ISO | Example numbers (illustration) |
|---|---|---|
| Effectiveness | Accuracy and completeness of users achieving their goals | Percentage of test participants who successfully completed the payment |
| Efficiency | Resources used compared to results; for example, time, effort, cost, materials | Seconds from opening the app to order delivery |
| Satisfaction | The extent to which users' physical, cognitive, and emotional responses meet their needs and expectations | Questionnaire score after task completion |
There are two important notes from this standard. First, ISO states that usability is not an attribute of the product, but rather a result of use. An application that is smooth for an experienced cashier can confuse an elderly person using it for the first time, because the context is different. This context consists of users, goals and tasks, resources, and the environment.
Second, ISO separates usability from user experience. UX is defined as the user's perception and response before, during, and after using something. Accessibility is also considered a separate outcome of use. Therefore, "UX equals comfort" is too narrow: comfort is just a part of satisfaction.
Ten heuristics: evaluation without waiting for respondents
Jakob Nielsen developed the heuristic evaluation method, which assesses interfaces using ten general principles. They are called heuristics because they are practical rules, not detailed guidelines. The ten are:
- Visibility of system status
- Match between system and the real world
- User control and freedom
- Consistency and standards
- Error prevention
- Recognition rather than recall
- Flexibility and efficiency of use
- Aesthetic and minimalist design
- Help users recognize, diagnose, and recover from errors
- Help and documentation
The usage is simple. Choose one flow, such as account registration, then navigate screen by screen and mark each violation. Heuristic number 9, for example, requires error messages to use plain language without codes, accurately state the problem, and offer a solution. The message "Error 0x80" fails on all three counts.
The result is a list of issues, not numbers. This is both its strength and its limitation.
SUS: numbers from real users
The System Usability Scale (SUS) is a 10-item questionnaire with scores ranging from 0 to 100 that measures users' perceptions of usability. MeasuringU collected data from over 10,000 responses across hundreds of products, with an average score of 68, precisely at the 50th percentile.
| SUS Score | Grade | Percentile |
|---|---|---|
| 80.8 and above | A | 90 and above |
| 65.0 to 71.0 | C | 41 to 59 |
| 51.7 to 62.6 | D | 15 to 34 |
| 51.6 and below | F | below 15 |
A score of 75 means better than 73 percent of products in that database. A score of 52 means worse than 85 percent. Users classified as promoters (most likely to recommend the product) typically give an average score of around 81, which is a high requirement.
SUS has limitations: it measures perception, so it does not indicate which screen has issues. Therefore, it is most useful as a comparison before and after improvements.
A logical workflow
Start with heuristics because they are inexpensive and can be done independently. Follow up with task testing on several users to measure effectiveness and efficiency. Conclude with SUS, so that satisfaction has a number that can be compared from release to release.
If you prefer to see the explanation directly, there is a short clip about the difference between UI and UX.
Sources
- ISO 9241-11:2018, Ergonomics of human-system interaction, Part 11: Usability: Definitions and concepts (ISO, second edition, March 2018; preview from SIST): https://preview.sist.si/sist-preview/63500/33c267a5a7564f298f02bbd65721a181/ISO-9241-11-2018.pdf
- Jakob Nielsen, 10 Usability Heuristics for User Interface Design (text of the ten heuristics, NN/g reference): https://principles.design/examples/10-usability-heuristics-for-user-interface-design
- Nielsen Norman Group, Heuristic Evaluation of User Interfaces (video, 2019): https://www.nngroup.com/videos/heuristic-evaluation/
- Jeff Sauro, 5 Ways to Interpret a SUS Score, MeasuringU, September 19, 2018: https://measuringu.com/interpret-sus-score/