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Focus groups are usually used to get input for the design process. A group of people from a specific user group are invited to change thoughts and feelings about certain subjects. A focus group will most likely give better results than a one-to-one interview, because the participants can help each other to get more and more ideas. On the other hand, if there are some very dominant members in the group they might skew the outcome. The group moderator needs to be experienced so that s/he can steer the discussion in a way that everyone can have her/his say. That way the group will give the best result.
Focus groups can also be arranged to evaluate usability of an existing application, for example. It then helps to discover how the system being tested differs from the user’s expectations. Also when evaluating with focus group, the benefit of interaction is clear. One person may not remember all the observations s/he has made but the memories might be triggered by something someone else says.