While the distinction between well-structured and illstructured problems is widely recognized in cognitive science, it has not generally been noted that there are often significant differences in the external representations which accompany the two problem types. It is here suggested that there is a corresponding distinction to be made between "well-structured" and "ill-structured" representations. Such a distinction is used to further differentiate diagrams into finer-grained types, loosely corresponding to sketches and drafting type diagrams, and it is argued that ill-structured, open-ended problems, like the preliminary phases of design problem solving, need "ill-structured" diagrammatic representations. Data from protocol studies of expert designers are used to support the thesis.
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