There may also be problems discovered in usability tests, which would have been hard to find by heuristic evaluators. The latter can be partly explained by realizing that instead of really using the system the evaluators are often able to only look at screen mock-ups. Evaluation is most effective when it is run by a researcher, who is specialized in both usability and the application domain. If it is impossible to find any, a good alternative is to have a non-domain expert, who is a usability specialist. No matter how experienced and gifted the evaluator is, a single evaluator can find only some of the usability problems in the user interface of the system.
Different evaluators find different problems.
One of the user interface specialists is conducting the session, the main purpose of which is to collect as many opinions as possible from users by discussing the screens. Users are encouraged to represent their viewpoints to the rest of the group at the very beginning of the discussion of each screen. Users' opinions are the major issue of the walkthrough and they should not be affected by specialists' own opinions. Usability test During a usability test, a test subject is asked to use the system performing a set of tasks. Subjects are encouraged to think aloud while performing tasks.