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A Computer-Aided Design System for general use must have a unique and powerful organization. Even the simplest of design problems involves the exercise of many disciplines and the carrying out of many types of activity. Since the area of applicability of the design system is to be essentially unlimited, we know from the beginning that the system itself must be very large and complex. Even though only a few of its features may be exercised on any given design problem, there is no way of predicting which portions of the system will be required nor how they will be used. Furthermore the designer or engineer who is using the system cannot be expected to be a computer programmer, and it must be possible for him to carry out his design function in a way which is natural to him, and without his being aware that the statements and actions that he performs are in fact constructing and executing large numbers of highly complex computer programs. Although to be sure the user must learn and become facile with the basic vocabulary and manipulations of the system, the system must be so designed that he finds his normal thought processes aided, augmented, and stimulated by the use of the system in such a way that he is able to think almost entirely at the concept level within his own field of interest, while at the same time carrying out data processing activities of extreme complexity.
Ross et al. (Tue,) studied this question.