ABSTRACT
Explanation and help facilities are among the most essential issues in the usability of expert systems. Explanation facilities include the ability of an expert system to provide answers to several types of question, such as questions concerning expert system behaviour ('how' questions) and questions requesting justifications ('why' questions). Facilities for on-line help, on the other hand, are an essential part of any complex software system: expert systems are no exception to this rule. In the area of statistical expert systems the importance of explanation and help facilities has been widely recognized—explanation has been noted as one of the necessary attributes of statistical expert systems (Hand, 1985), and some statistical expert systems adapt their help to the users (Nelder, 1988). Moreover, the characteristics of statistical analysis give rise to new demands for explanation, for example explaining new statistical terms via lexicon (Gale, 1986; Hietala, 1990).
