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Usability

Concept

Effective, efficient, satisfying: the three dimensions of usability

Defined in the ISO 9241-11 standard, usability measures whether a product lets its users reach their goals with effectiveness, efficiency and satisfaction.

Usability is one of the dimensions of user experience, but not the only one. Good usability contributes to a good user experience, but it is not enough on its own: desirability is one of the major complements to usability.

What is it for?

  • Assessing whether a product lets people reach their goals without excessive friction
  • Measuring the impact of an interface on user performance and satisfaction
  • Identifying obstacles to use before or after development

What you need to know

  • It is not the whole user experience: it is one dimension of it, one that sits alongside desirability in particular
  • It is not accessibility: accessibility makes the product usable by everyone, usability makes it low-effort and pleasant to use
  • It is never measured in the abstract: it is always defined for a given user, a given task and a given environment

The 4 pillars of usability

01

Effectiveness

Corresponds to a user's ability to reach the goal they had set. It is measured by a success rate on a given task: how many users complete their purchase, finish their sign-up, or find the information they were looking for. A success can be complete or partial, users sometimes reach their goal without taking the path the product had planned, which is still a signal to take into account. A high effectiveness rate, on the other hand, says nothing about the effort it took to get there.

02

Efficiency

Corresponds to the level of effort the user has to put in to reach their goal. It is measured by the time spent on the task, the number of steps or clicks required, the number of errors made along the way. It can also be measured through self-report, by asking the user directly how demanding the task felt. A product can be effective without being efficient: the user reaches their goal, but at the cost of a journey twice as long as it needs to be.

03

Satisfaction

Corresponds to the impression the user is left with once the interaction is over. It is an emotional dimension, distinct from task performance alone. It is measured through standardised questionnaires such as the SUS, or more simply through satisfaction scales and verbatim comments collected on the spot. A task can be successful and quick while leaving a frustrating memory, or the other way round, which often makes it the most telling indicator of future attachment to the product.

04

The situated context of use

Usability is never measured in the abstract, it is always defined for a given user, a given task and a given environment, and that is what the ISO 9241-11 standard points out. The user's level of expertise, the device used, or the conditions of use (on the move, in noise, under pressure) change what is genuinely usable. A product judged smooth in the lab can stop being smooth in real conditions: poor connection, one hand busy, divided attention. That is why user testing has to get as close as possible to real conditions of use, or it risks measuring only surface-level usability.

Frequently asked questions

Related methods

UX audit

Systematic, detailed analysis of an interface (web, mobile, business tool) against a set of criteria, used to surface friction points and produce illustrated recommendations.

Read more about UX audit

User testing

User testing has target users assess the usability of your product or service. Depending on the goals, several approaches exist.

Read more about User testing

Ergonomics

Ergonomics adapts tools to human capabilities. Born in factories, it now applies to interfaces through cognitive ergonomics and UX design.

Read more about Ergonomics

Nielsen's heuristics

Nielsen's 10 heuristics let an expert evaluate an interface without users. The most widespread method for diagnosing usability.

Read more about Nielsen's heuristics

Bastien and Scapin's criteria

Bastien and Scapin propose 8 ergonomic criteria and 18 sub-criteria. This French academic framework is more rigorous than Nielsen's heuristics.

Read more about Bastien and Scapin's criteria

Digital accessibility

Digital accessibility makes products usable by everyone. In the digital field, the RGAA is the reference on the matter.

Read more about Digital accessibility

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