The dynamics of psychomotor skill development and its negative, poor performance and the emergence of errors, is a pervasive consideration in MMS design. The highest level of psychomotor skill is attained by a process of successive organization of perceptions (SOP), whereby operators fully familiar with the dynamics of the machine under their control and the appropriate responses to the input signals can reorganize the system, adapt their behavior to create a repertory of special responses, and then select from this repertory the appropriate response for system performance. In closed-loop control this response may be anticipatory and compensate for inherent human control lags. In an open-loop mode the response may be programmed, as in executing the sequential actions in preparing to stop a car on approaching a red light. Skilled performance can degrade precipitously if inappropriate cues contaminate the perceptual organization or cause it to regress; training in cue recognition is essential.
The level of skilled performance achieved and its variability is affected by individual abilities, training, motivation, attention, workload, work scheduling, the social environment of the activity, and impairment due to substance abuse, stressors, or fatigue. By adhering to human factors findings in the selection and positioning of information displays, controls, monitors, keyboards, seating, and accommodating the physical dimensions of the operator, the designer can make sustained desirable performance more likely, and thwarted or unintended responses less likely. Unintended responses such as misreading a display or inadvertently reaching for the wrong switch result in errors characterized as slips. The potential for catastrophic accidents in the operation of nuclear power plants, chemical plants, aircraft, and oil tankers emphasizes the need to design against such slips and to be able to estimate the reliability of both the MMS tasks and the total system. By ascribing human error probabilities to slips and other deteriorations in performance and developing detailed event trees for the many complex MMS tasks, Swain and Guttman were able to estimate human reliability in much the same way that the reliability of inanimate devices has been estimated. Although their numerical reliability estimates cannot always be precise, they enable comparisons to be made of the MMS’s reliability under alternative physical configurations, training disciplines, and institutional policies.
A different type of error in behavior arises from mistakes in the human’s thought processes. Mistakes are similar to errors arising from perceptual illusions in their inappropriate reliance on expectations and experiences. When the error is a mistake, the human operator has misinterpreted the situation, and reliability estimates are not feasible. The mental processes leading to mistakes have been developed from detailed reconstructions of the human actor’s thought processes and behavior in accidents and near accidents. The basic sources are accident reports, anecdotes, interviews with survivors, and skilled introspection on the part of the analyst. If the mistake is made under time pressure, it is difficult for the operator to reexamine the situation, sort out misconceptions, and examine options in a deliberate manner. Ergonomic design can make mistakes less likely and recovery more likely by reorganizing the information displays. A more direct approach to lessening the incidence of mistaken intentions is to design redundancy into the MMSs and to develop training and personnel selection procedures which emphasize the search for options and discourage perseverative mistaken thought processes.