shifts between manual control and resolution advisory coincided with traffic pattern shifts, giving the appearance that the adaptation was triggered by the traffic increase or decrease. Compared with the static automation conditions, the adaptive condition was associated with workload benefits, particularly under high traffic load. There was also a trend for monitoring to be better in the adaptive condition, compared with the two static conditions, consistent with the previously described study by Parasuraman, Mouloua, and Molloy (1996).

Despite these performance benefits, adaptive systems may not be free of some costs. For example, if the adaptive logic on which the system is based is oversensitive to the eliciting criteria, then the system may oscillate between automated and manual control of a task at frequent intervals. There is evidence that performance costs can occur if the cycle time between automated and manual control of a task is very short, particularly if the operator has no control over function changes (Hilburn, Parasuraman, and Mouloua, 1995; Scallen et al., 1995).

The question of operator control leads to the issue raised by Billings and Woods (1994) on adaptive versus adaptable automation. Very little empirical work has been done on this issue. Hilburn et al. (1993) had individuals perform a multitask flight simulation with the ability to turn automation on or off whenever they chose (adaptable automation). The times that automation was invoked or turned off were recorded and presented as the output of an intelligent adaptive system to another group of individuals in a yoked-control design modified from one used by Liu et al. (1993). Overall performance was superior for the adaptable automation group compared with the adaptive automation group, consistent with the findings of Billings and Woods (1994), although automation benefited both groups.

In these and other studies on adaptive automation, function changes involved allocation of entire tasks to automation or to human control. As mentioned earlier, another possibility is to partition tasks—that is, to allocate subtasks. Partitioning may lead to performance costs if tasks are partitioned in a nonmodular way (Gluckman et al., 1993). Vortac and Manning (1994) also found performance costs of partitioning in an air traffic control context. They suggested that automation benefits will accrue only if entire behavioral modules are allocated to automation. Finally, adaptive systems may not necessarily be immune from operator errors arising from misunderstanding or lack of awareness of the activities of the automation at a particular time (Sarter and Woods, 1995b). Given that adaptive systems will probably be granted higher levels of autonomy than current automation, automation-related surprises may occur, particularly if the system is slow to communicate intent to the human. Bubb-Lewis and Scerbo (1997) have considered ways in which human-computer communication can be enhanced in adaptive systems, but problems in coordination and communication remain potential concerns with such systems. It remains to be seen whether these potential

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