A person starts with the psychological variables of goals and intentions. Since the task is performed on a physical system, the person must interpret the physical variables into terms relevant to the psychological goals and translate the psychological intentions into physical actions upon the mechanisms. The variables of interest to the user interact with the physical variables, which can result in problems with mapping, ease of control and evaluation.
The following are aspects of a task. (1) A goal is the state the person wishes to achieve; an intention is the decision to act so as to achieve the goal. (2) The specification of the action sequence is the psychological process of determining the psychological representation of the actions that are to be executed by the user on the mechanisms of the system. (3) Mapping from psychological goals and intentions to action sequence. In order to specify the action sequence, the user must translate the psychological goals and intentions in the desired System State, then determine what settings of the control mechanisms will yield that state, and then determine what physical manipulations of the mechanisms are required. The result is the internal, mental specification of the actions that are to be executed. (4) The physical state of the system is determined by the values of all its physical variables. (5) The Control Mechanisms are the physical devices that control the physical variables. (6) The Mapping between the physical mechanisms and System State is the relationship between the settings of the mechanisms of the system and the System State. (7) The interpretation of the system state. The relationship between the physical state of the system and the psychological goals of the user can only be determined by first translating the physical state into psychological states (perception), then interpreting the perceived system state in terms of the psychological variables of interest. (8) Evaluation the outcome. Evaluation of the System State requires comparing the interpretation of the perceived System State with the desired goals. This often leads to a new set of goals and intentions.
The gap from goals to physical system is referred to as the gulf of execution and is bridged in four segments: intention formation, specification of the action sequence, execution of action and contact with the input mechanisms of the interface. The gap from physical system to goals is called the gulf of evaluation and is bridged by interface display, interpretation and evaluation. The practical implication of these two gulfs is that the system has to be moved closer to the user or the user must be moved closer to the system.
The conceptual model is composed of the design model, the user's model and the system image. Tools, though helpful, have problems because they can require too much skill from the user.
The prescriptions for design: (1) Create a science of user-centered design. (2) Take interface designs seriously an independent and important problem. (3) Separate the design of the interface from the design of the system. (4) Do user-centered system design by starting with the needs of the user.