descriptions, the evaluators might not fully realise the heterogeneity of the user group or their walkthrough might be anchored to their own experience with the system. Each of the four questions drives the evaluator to think of the user's behaviour in a certain situation. When the fictive user description becomes too fuzzy or lacks details to judge the user's behaviour, the evaluator unintentionally substitutes the description with a particular user much like herself/himself. Thus, evaluators tend to produce success stories if they imagine themselves having no troubles using the feature in question, and they report problems when they imagine themselves having troubles in the particular situation. In this sense a single evaluator using CW resembles an evaluator performing a thinking-aloud study with one user, namely himself/herself.
Wharton et al. ( 1994) state that CWs can be performed by individual evaluators as well as by groups of co-operating evaluators. For inexperienced CW evaluators our study strongly indicates that several evaluators are necessary to achieve a performance that is acceptable for practical use of the CW technique. Additional studies are required to learn how more experienced evaluators perform and to study more closely why we see these individual differences.
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