Users' mental models impact interface usability

What users believe they know about a UI strongly impacts how they use it. Jakob Nielsen's article on mental models touches on the theory, but also gives some great examples of the strange (or should that be predictable?) ways people use interfaces based on what they believe is happening.

The bottom line here, I think, is consistency. People expect things that look similar to work the same.

Jakob's Law of the Internet User Experience: Users spend most of their time on websites other than yours. Thus a big part of customers' mental models of your site will be influenced by information gleaned from other sites.

There's a great example of the confusion people have between search boxes and browser address bars.

I've seen a classic example on the University website lately while analysing search terms. Our search box has some text in it by default - "Search term" - which the user is expected to delete or type over before searching. Guess what he number 1 term searched for on the University website? Yep - it's 'Search term'. Thousands of people every month click the 'Go' button without entering what it is they're looking for in the search box.

Mental Models - article by Jakob Nielsen