Although the definition of "mental work" remains elusive, systematic means/ methods should be considered for gaining evidence about interactive language features requiring more/less effort of the human mind. The suggested approach employs a structuring of the user's conceptual reference spaces into sets of "action primitives", peculiar to the type of computer-aided task involved. An interactive command language can then be regarded as the range of some transformation on the user's set of action primitives. The nature and efficiency of that transformation, in conjunction with the inherent number of mental association links, are hypothesized to have direct relationships to the level of required mental work. The user's delay or "think time", expended immediately preceding command utilization, is one measurable quantity that should be useful as a work level indicator. s. TREU carried out in a very deliberate and (preferably) deterministic manner, viewing man and computer as a pair of different (although in some respects similar) information processors.
FJMPHASIS ON COMMAND LANGUAGE
Regardless of which interactive computer application is involved, interface design should presuppose the designer's recognition of the strengths and weaknesses not only of specific computers but also of the particular users. This means, in general, that all of the significant aspects or characteristics must be appropriately understood and taken into account (Treu, 1971). However, in view of the relative lack of all-encompassing research and resulting "total interface" design guidelines, this is certainly not an easy task. Hence, as is often true, instead of attempting to tackle the whole problem area, it can be more fruitful to concentrate on one important element only-without losing sight of the larger picture.
The focus of this paper is on interactive command language. That is viewed as a crucial component of the conceptual framework presented to the user, in that it represents a very basic means for potentially enabling "compatibility" in user-computer communication.
Many different kinds of interactive command languages exist, as is well known. Some are very explicit in nature, some carry implicit command actions. Some relate strictly to the computer's operating systems, many others enable communication with the various subsystems or processes available in operating system environments. Some are very procedure-oriented, others are problem-oriented. Collectively they represent a conglomeration of mostly interincompatible, often cumbersome and, in general, quite confusing means of telling computers what to do. Licklider 0968) gave a vivid portrayal in saying:
The field of man-computer interaction languages is like the field of Babel after the tower builders displeased God.
While that may sound overly pessimistic, especially in view of a number of currently promising research activities involving automated language processing (Walker, 1973), it still has considerable pertinence for most users attempting to communicate with most computer systems in operation today.
MENTAL WORK CRITERION
Another way of depicting different types of interactive command languages is by means of the spectrum shown in Fig. 1.