An interface is designed for people performing tasks in a particular context of use.

A useful description separates three things:

• users: relevant abilities, experience, language and constraints;
• tasks: goals people are trying to achieve, including frequency and consequences of mistakes;
• context: devices, environment, interruptions, collaboration and other conditions surrounding use.

A workflow that is acceptable once per year may be intolerably slow when repeated hundreds of times per day. A control that works at a desk may fail when the user wears gloves or cannot look continuously at the display.

Design should therefore be based on representative tasks rather than an abstract idea of an “average user”. Observing real work, examining existing procedures and breaking goals into steps can reveal requirements that feature lists miss.

The context of use defines which usability trade-offs actually matter.