Situation Awareness and Workload: Birds of a Feather?

1. SUMMARY from a set of desired states or goals, the dynamics of the controlled system and the interactions between system variables. This knowledge forms an internal representation or mental model of the process to be controlled. The concept of a mental model, which the operator develops and draws upon when making operational decisions, is central to the idea of situation awareness, and has become an aspect of particular concern to engineers and behavioural scientists involved in the development of complex humanmachine systems. In this paper it is argued that an hierarchical information processing model, with a basis in perceptual control theory, provides the necessary framework for interpreting a large, unfocused empirical literature on the topics of workload and situation(al) awareness (SA). The fundamental importance of situation awareness will emerge in considering the role of the mental model in providing the reference signal for a closed loop perceptual control system. It will be asserted that those aspects of the mental model generally covered by the SA rubric result from high level information processing activity that requires spare capacity to service. Increasing time pressure (workload) reduces the capacity available for this activity. An experiment in the application of a workload scale (NASA TLX) and a situation awareness metric (SART) to a simulated air traffic control environment is cited. It will be shown that the situation awareness scale taps largely into the workload side of the equation rather than the SA side. Implications for the measurement of SA will be drawn. While measures of performance and workload have been the typical metrics employed for determining the efficacy of human-machine interactions, there are certain conditions under which these measures are limited (see, for example, the work of Yeh and Wickens [1]). Take, for example, a situation in which the optimum strategy for an operator is to simply wait and monitor system variables before deciding whether or not to take action. In this situation there may be no overt performance to measure but cognitive load may be high. Further, consider a situation where an operator is flooded with activities, or the converse, where workload is relatively low and the operator is performing a passive monitoring role. Each of these scenarios, though arguably opposite in terms of their levels of workload, may produce a state of low situation awareness. In the former case the operator may have little spare capacity to develop a mental model while in the latter case the operator may be outof-the-loop and lacking both relevant information, and a feel for the system dynamics which are essential to building the knowledge state that would allow an effective intervention. Because of the potential difficulty in determining operator effectiveness under these types of conditions, one might speculate that the concept of the mental model may help provide relevant information about an operator's potential to perform effectively in certain types of complex systems. 2. INTRODUCTION Consider the following statement