The guidelines and principles described in the previous sections are often
devoted to simplifying the users' tasks. Users can then avoid routine, tedious,
and error-prone tasks and can concentrate on making critical decisions, coping
with unexpected situations, and planning future actions (Sanders and
McCormick, 1993). (Box 2.3 provides a detailed comparison of human and
machine capabilities.)
The degree of automation increases over time as procedures become more
standardized and the pressure for productivity grows. With routine tasks,
automation is desirable, since the potential for errors and the users' workload
are reduced. However, even with increased automation, designers can still offer
the predictable and controllable interfaces that users often prefer. The human
supervisory role needs to be maintained because the real world is an open system
(that is, there is a nondenumerable number of unpredictable events and system
failures). By contrast, computers constitute a closed system (there is only a denumerable
number of normal and failure situations that can be accommodated in
hardware and software). Human judgment is necessary for the unpredictable
events in which some action must be taken to preserve safety, to avoid expensive
failures, or to increase product quality (Hancock and Scallen, 1996).
For example, in air-traffic controt common actions include changes to altitude,
heading, or speed. These actions are ,veil understood and can potentially
be automatable by a scheduling and route-allocation algorithm, but the controllers
must be present to deal with the highly variable and unpredictable
emergency situations. An automated system might deal successfully with high
volumes of traffic, but what would happen if the airport manager closes runways
because of turbulent weather? The controllers would have to reroute
planes quickly. Now suppose that one pilot requests clearance for an emergency
landing because of a failed engine, while another pilot reports a passenger
needing treatment for a potential heart attack. Human judgment is
necessary to decide which plane should land first, and how much costly and
risky diversion of normal traffic is appropriate. Air-traffic controllers cannot
just jump into the emergency; they must be intensely involved in the situation
as it develops if they are to make an informed and rapid decision. In short,
many real-world situations are so complex that it is impossible to anticipate
and program for every contingency; human judgment and values are necessary
in the decision-making process.