A mental model is a person’s internal, personalised, contextual understanding of ‘how something works’. This understanding may, or may not accurately match the reality of how that thing actually works.
A person draws on an existing mental model to help them to figure out how to use a website. If a single model ‘doesn’t quite fit’ (and there is sufficient incentive for them to persevere), then they will adapt or combine different mental models to help them to progress.
Mental models are formed either through direct, or shared experience of - what the individual believes to be - related content, services or systems.
An effective interface design will often draw on a model (or metaphor) that is familiar to the intended user group.
For example, online shopping systems encourage people to complete the process as they would shopping at a supermarket.
A person’s mental models are reinforced when they seem to accurately ‘match’ or ‘explain’ their direct experience, and when they can confidently use an existing understanding to predict future outcomes.
Usability evaluation provides methods that can help externalise a users’ mental models. Especially valuable are think-aloud protocols where a person verbalises what they believe they are doing, as they use a website.
What users believe they know about a UI strongly impacts how they use it. Mismatched mental models are common, especially with designs that try something new.