The assessment of operator workload has a vital impact on the design of new human-machine systems. By evaluating operator workload during the design of a new system, or iteration of an existing system, problems such as workload bottlenecks and overload can be identified. As the human operator is a central part of a human-machine system, the correction of these problems is necessary for the operation of safe and efficient systems.
An operating budget may include estimates of the expected workload for a specific activity.
This theory was also used to determine horse power (hp), which was defined as the amount of work a horse could do with a given load over time. The wheel that the horse turned in Watt's original experiment put a certain load on the horse's muscles, and the horse could do a certain amount of work with this load in a minute. Provided the horse was a perfect machine, it would be capable of a constant maximum workload, so increasing the load by a given percentage would result in the possible work done decreasing by the same percentage, so that it would still equal "1 hp". However, horses are not perfect machines and over short time periods are capable of as much as 14 hp, and over long periods of exertion output an average of less than 1 hp.
The theory can also be applied to automobiles or other machines, which are slightly more "perfect" than animals, making a car heavier for instance, increases the load that the engine must pull, likewise making it more aerodynamic decreases drag, which acts as a load on the car as well. Torque can be thought of as the ability to move load, and the revs are how much work it can do with that load in a given amount of time. Therefore torque and revs together create kilowatts, or total power output, which can be related to the "workload" of the engine/car, or how much work it can do with a given amount of load. As engines are more mechanically perfect than animals' muscles, and do not fatigue in the same way, they will confirm much more closely to the formula that if you apply more load, they will do less work, and visa versa.
Another aspect to workload is the mathematical predictive models used in human factors analysis; generally to support the design and assessment of safety-critical systems.
There is no one agreed definition of workload and consequently not one agreed method of assessing or modelling it. One example definition by Hart and Staveland (1988) describes workload as “the perceived relationship between the amount of mental processing capability or resources and the amount required by the task”. Workload modelling is the analytical technique used to measure and predict workload. The main objective of assessing and predicting workload is to achieve evenly distributed, manageable workload and to avoid overload or underload.
Wickens’ (1984) multiple resource theory (MRT) model is illustrated in figure 1:
Figure 1: Wickens’ Multiple Resource Theory (MRT) Model
Wickens MRT proposes that the human operator does not have one single information processing source that can be tapped, but several different pools of resources that can be tapped simultaneously. Each box in figure 1 indicates one cognitive resource. Depending on the nature of the task, these resources may have to process information sequentially if the different tasks require the same pool of resources, or can be processed in parallel if the task requires different resources.
Wickens’ theory views performance decrement as a shortage of these different resources and describes humans as having limited capability for processing information. Cognitive resources are limited and a supply and demand problem occurs when the individual performs two or more tasks that require a single resource (as indicated by one box on the diagram). Excess workload caused by a task using the same resource can cause problems and result in errors or slower task performance. For example, if the task is to dial the phone then no excess demands are being placed on any one component. However, if another task is being performed at the same time that makes demands on the same component(s), the result may be excess workload.
The relationship between workload and performance is complex. It is not always the case that as workload increases performance decreases. Performance can be affected by workload being too high or too low (Nachreiner, 1995). Sustained low workload (underload) can lead to boredom, loss of situation awareness and reduced alertness. Also as workload increases performance may not decrease as the operator may have a strategy for handling task demands.
Wickens’ theory allows system designers to predict when:
McCracken and Aldrich (1984), like Wickens, describe processing not as one central resource but several processing resources: visual, cognitive, auditory, and psychomotor (VCAP). All tasks can be decomposed into these components.
They developed rating scales for each of the VCAP components, which provide a relative rating of the degree to which each resource component is used.
Joseph Hopkins (unpublished) developed a training methodology, where the background to his training theory is that complex skills are, in essence, resource conflicts where training has removed or reduced the conflicting workload demands, either by higher level processing or by predictive time sequencing. His work is in effect based on Gallwey (1974) and Morehouse (1977). The theory postulates that the training allows the different task functions to be integrated into one new skill. An example of this is learning to drive a car. Changing gear and steering are two conflicting tasks (i.e. both require the same resources) before they are integrated into the new skill of “driving”. An experienced driver will not need to think about what to do when turning a corner (higher level processing) or alternatively may change gear earlier than required to give sufficient resources for steering round the corner (predictive time sequencing).
With any attempt at creating a workload model the process begins with understanding the tasks to be modelled. This is done by creating a task analysis that defines:
Each task must be defined to a sufficient level to allow realistic physical and mental workload values to be estimated and to determine which resources (or combination of resources) are required for each task – visual, auditory, cognitive and psychomotor. A numerical value can be assigned to each based on the scales developed by McCracken and Aldrich.
These numerical values against each type of resource are then entered into the workload model. The model sums the workload ratings within each resource and across concurrent tasks. The critical points within the task are therefore identified. When proposals are made for introducing new devices onto the current baseline activities the impact of this can then be compared to the baseline. Possibly one of the most advanced workload models was developed by K Tara Smith (2007): this model integrated the theories of Wickens, McCracken and Aldrich and Hopkins to produce a model that not only predicts workload for an individual task but also indicates how that workload may change given the experience and training level of the individuals carrying out that task. Workload assessment techniques are typically used to answer the following types of questions: Eisen, P.S and Hendy, K.C. (1987):