Evidence rating
Effect score (E): 5/5 Evidence for the underlying principle or measurement method.
Transfer score (T): 4/5 Evidence for reliable results in digital product practice.
Evidence level: High
[Plausible based on UX research] Always assess usability for defined users, goals, and contexts of use.
Scale and rating rules
Effect score: 5 established · 4 robust · 3 context-dependent · 2 weak · 1 preliminary.
Transfer score: 5 practice-proven · 4 field-supported · 3 partially supported · 2 plausible · 1 hypothesis.
Without a robust scientific or normative source, E is capped at 3. Blog and agency opinions support at most T=2. If evidence conflicts, the more conservative score applies until justified otherwise.
Evidence level: High = E ≥ 4 and T ≥ 3; Moderate = E ≥ 3; Preliminary = E ≤ 2 or T ≤ 1.
What Is Usability?
Usability is how well specified users can achieve specified goals with a system, product or service in a specified context of use. Under ISO 9241-11, it is assessed through effectiveness, efficiency and satisfaction.
Definition (ISO 9241-11)
Usability is the extent to which a system, product or service can be used by specified users in a specified context of use to achieve specified goals effectively, efficiently and satisfactorily.
What does usability mean in context?
Usability is a key quality characteristic of interactive systems. It does not describe whether a product is abstractly “easy,” but whether particular users can achieve particular goals under actual conditions. These include prior experience, tasks, devices, environment, time pressure and available support. The same application may work well for experienced staff at a desktop but cause difficulty for first-time users on a small screen under time pressure.
The three usability criteria in ISO 9241-11
ISO 9241-11:2018 describes usability through three related criteria:
1. Effectiveness
Effectiveness is the extent to which users achieve their goals accurately and completely. For an online form, this could mean checking whether the intended request was submitted correctly.
2. Efficiency
Efficiency relates the results achieved to the resources used, such as time, steps, or mental and physical effort. If two people reach the same goal but one needs considerably more time and detours, the experience is less efficient for that person.
3. Satisfaction
Satisfaction covers users’ responses and evaluations resulting from use. It can be explored through questions about clarity, confidence or perceived effort. Task completion alone does not reveal whether use felt pleasant or frustrating.
ISO 9241-11 and ISO 9241-110: definition and design
The ISO 9241 series provides a framework for designing and evaluating interactive systems. Two parts are especially relevant to usability:
- ISO 9241-11:2018 – usability: defines usability through effectiveness, efficiency and satisfaction in relation to specified users, goals and a context of use. It helps determine what to assess.
- ISO 9241-110 – dialogue principles: describes principles for designing interaction, including suitability for the task, self-descriptiveness, conformity with user expectations, learnability and controllability. It helps designers create interfaces that can be usable in their context.
- ISO 9241-210 – human-centred design: describes a design process that continuously considers users, tasks and the context of use.
In short, ISO 9241-11 describes and evaluates the outcome, while ISO 9241-110 provides dialogue design principles and ISO 9241-210 addresses the human-centred development process. The parts complement one another but are not interchangeable.
Usability and user experience (UX): what is the difference?
Usability and UX are closely related, but they are not the same. Usability focuses on whether specific tasks can be completed effectively, efficiently and satisfactorily in context. User experience (UX) covers people’s broader perceptions and responses before, during and after use, including expectations, emotions, trust and the value they assign to the experience.
| Comparison | Usability | User experience (UX) |
|---|---|---|
| Key question | Can users achieve their goals effectively, efficiently and satisfactorily? | How do people perceive and experience the product before, during and after use? |
| Focus | Specific tasks, ease of use and context | Holistic, subjective and emotional experience |
| Evaluation | For example, task success, completion time, errors and satisfaction ratings | For example, expectations, emotions, trust, perception and experience |
| Relationship | A core quality characteristic and important foundation | A broader concept to which usability contributes |
Good usability is an essential foundation for good UX: if people cannot complete a core task, their overall experience often suffers. But smooth operation alone does not guarantee positive UX—for example, if a product fails to meet people’s needs or inspire trust.
Why is usability important?
- Economical: Good usability reduces support costs and increases conversions.
- Psychological: It promotes cognitive relief, motivation and trust.
- Ethical: It prevents frustration, errors and potential risks of use.
- Normative: It is part of many standards (e.g. ISO 9241, EN 301 549).
How can usability be evaluated?
Create representative tasks from the context of use and observe whether users complete them accurately and fully, how much time and effort they need, and how they rate the experience. Task success rate, time-on-task, errors and satisfaction surveys examine different aspects; no single metric describes usability in full. Heuristic evaluations can identify issues early, but important decisions should ideally be checked with user testing.
Conclusion: usability is context-dependent
Usability asks whether specified users can achieve their goals with a system, product or service in a particular context effectively, efficiently and satisfactorily. ISO 9241-11 provides the definition and criteria; ISO 9241-110 adds interaction design principles. Usability is a measurable, designable foundation for good UX, but not the whole experience.
Further Reading
- ISO 9241-11:2018
- ISO 9241-11 explained
- ISO 9241-110 dialogue principles
- Jakob Nielsen’s Usability Heuristics
Frequently asked questions (FAQ)
What does usability mean?
How does ISO 9241-11 define usability?
What do effectiveness, efficiency and satisfaction mean?
What is the difference between usability and user experience?
What does ISO 9241-110 cover?
How do you measure usability?
Is usability an inherent property of a product?
Science corner: evidence data
- Setting
- Standard definition and evaluation of concrete usage scenarios
- Replication status
- Theoretically or normatively grounded
- Evidence level
- High (E=5, T=4)
- Key sources (APA)
-
- International Organization for Standardization. (2018). Ergonomics of human-system interaction—Part 11: Usability: Definitions and concepts (ISO Standard No. 9241-11:2018). Open source
- Bevan, N., Carter, J., & Harker, S. (2015). ISO 9241-11 revised: What have we learnt about usability since 1998? In M. Kurosu (Ed.), Human-computer interaction: Design and evaluation (Lecture Notes in Computer Science, Vol. 9169, pp. 143–151). Springer. https://doi.org/10.1007/978-3-319-20901-2_13
- Frøkjær, E., Hertzum, M., & Hornbæk, K. (2000). Measuring usability: Are effectiveness, efficiency, and satisfaction really correlated? Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, 345–352. https://doi.org/10.1145/332040.332455
- Hornbæk, K. (2006). Current practice in measuring usability: Challenges to usability studies and research. International Journal of Human-Computer Studies, 64(2), 79–102. https://doi.org/10.1016/j.ijhcs.2005.06.002
- Review date / version
- 2026-11-01 · 1.1
- Next review
- 2027-11-01 (Review cycle: 12 months)
Last modified: 1 November 2026