What Is Usability?

Jakob Nielsen defines usability as a quality attribute that assesses how easy user interfaces are to use. The word also refers to the methods for improving ease of use during the design process. Nielsen breaks it down into five quality components: - Learnability: how easy it is to accomplish basic tasks the first time. - Efficiency: how quickly tasks can be done once the design has been learned. - Memorability: how easily users regain proficiency after a period away. - Errors: how many errors users make, how severe they are and how easily users recover. - Satisfaction: how pleasant the design is to use.

In the wording quoted by the W3C, the ISO 9241-11 standard defines usability as the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use. The current edition dates from 2018, was confirmed in 2023 and treats usability as an outcome of use. The standard does not describe design or evaluation methods.

Nielsen separates usability from utility, which is whether the design has the features users need. Together they decide whether something is useful: it matters little that something is easy if it does not do what you want.

The basic method is user testing. According to Nielsen it has three parts: recruit representative users, ask them to perform realistic tasks and observe where they succeed and where they struggle, without helping them. Participants are often asked to think aloud as they work.

NN/g distinguishes two kinds of test. Qualitative testing looks for problems and is the more common. For a single user group, NN/g recommends five participants. In 2000 Nielsen calculated that each user uncovers 31 % of the problems on average and that five users find about 85 %. Quantitative testing collects metrics such as task success and time on task, is suited to comparing versions and needs more participants; NN/g points to 20 users.

Without users, an expert can review the interface against Nielsen's ten heuristics, published in 1994. Nielsen himself calls them broad rules of thumb rather than specific usability guidelines, so a heuristic review does not replace testing.

On the web, Nielsen writes, usability is a condition for a site to work at all. If a site is hard to use, if the home page does not say what is on offer or if users get lost, they leave. Nobody reads a manual to understand a website.

Timing matters too. Nielsen advises testing before a redesign, with prototypes and during implementation. Many problems are structural, and if they surface only once the design is fully built, fixing them means rebuilding much of it.

Usability is one part of user experience, which covers the whole relationship with the company and its product. The user interface is the object being evaluated, and web accessibility makes sure that people with disabilities can use it on equal terms. All three have their own entry in this glossary.

• Run several small tests rather than one large one: Nielsen proposes three studies with five users each instead of one with fifteen.
• Recruit users who resemble the real audience, not colleagues from the team.
• Set realistic tasks and do not explain the interface during the session.
• Test the current design before a redesign, to know what to keep.
• Measure task success and time on task when two versions need comparing.

• Using a focus group to evaluate a design: Nielsen warns that what people say is misleading and that you have to watch what they do.
• Helping participants when they get stuck, which contaminates the result.
• Leaving testing until the end, when structural problems can only be fixed by rebuilding.
• Treating a heuristic review without users as proof of usability.
• Assuming that a usable site is also accessible: the W3C notes that usability practice often does not consider people with disabilities.