The most frequent complaint of operators in modern computer-based control rooms is that there just are not enough video display units (VDUs). In this paper we examine the basis for this concern and try to understand the technical and historical reasons for this complaint, and its implications for the design of complex human-machine systems, including the number of VDUs in the control room. The overall aim of our work is to develop human factors guidance for the review of computer-based and modernized control rooms in nuclear power plants. As part of these efforts we have conducted literature reviews and studies using both simulators and actual systems in a broad range of industries, including process control, aerospace, medical, and others. Our findings reflect the general complaint of operators across all these industries: there just are not enough VDUs in the control room. We conclude that there are three primary reasons for this complaint. First, as part of a workload management strategy, operators frequently avoid interface management tasks and do not access all the information available, preferring instead to use a fixed set of familiar displays that provide much (but not all) of the information needed. Performance thereby becomes data limited and operators complain that they do not have a sufficient number of VDUs to set up in the early phases of a high-workload period so they can get all the information they need. Second, display designs are typically not designed with operator tasks in mind. The most common method of representing information is by functions and systems. Since tasks typically cut across many systems, operators need many displays. Thus, to make task performance easier operators need additional VDUs. Finally, there is a differing ''concept of operations'' between designers and operators. Modern computer-based control rooms are designed with vast amounts of data, available through hundreds of displays, viewed by the operator through a limited number of display devices. Designers expect that operators will use the flexibility of the computer-based interfaces to configure them in such a way that they are ideally tailored to the unique demands of the current situation. However, operators usually do not do that and instead configure the interfaces in a spatially dedicated way. Thus, while the number of VDUs may seem reasonable to the designer, it is not to the operator who is attempting to minimize the interface management aspects of workload. The implications of these findings for design are discussed in terms of the need for a method for determining the number of displays, task-relevant displays, data-dense displays, and enhanced interface management design and training.
