In this chapter we have examined some of the issues involved in defining and measuring performance on Earth and in space. The measurement of performance under space-operational conditions poses many problems. Because of the complexity of the tasks and the environment, it is seldom easy to investigate the many basic human abilities that must work together for a mission requirement to be completed successfully; nor is it easy to understand and predict the impact of the myriad of overlapping environmental stressors that may affect that performance.
For example, navigation of a spacecraft may require the execution of a number of human abilities including arm-hand steadiness, finger dexterity, hand-eye coordination, perceptual speed, and rapid reaction time, all happening with such fluidity and coordination that to assess the individual level of each factor at a given moment is not feasible. Furthermore, even if a change in the quality of vehicle control is observed, it may be difficult to determine whether this change is due to fatigue, space sickness, increased arousal, or loss of sleep, or to some combination of these and/or other factors.
Investigators have sought to develop means outside the operational setting to aid in the understanding of processes present within  the operational setting. One approach is the discrete-task assessment technique. A task is selected which purports to measure some aspect of performance believed to be important in the operational setting. For example, rotary pursuit testing may be used to measure tracking ability, or visual monitoring may be used to measure the subject's vigilance. This approach is low cost, permits precise measurement of performance, and can be used to investigate the effects of individual environmental stressors. However, there remain significant limitations and important research questions regarding this approach. Frequently there is little similarity in the type of task employed in the laboratory to measure a certain ability and the task requiring that ability in the operational setting. The predictive validity of individual laboratory tests as they relate to space missions needs to be determined.
The discrete-task technique frequently fails to tap the more fluid, simultaneous demands of the operational setting. For this reason, some investigators have relied on multiple-task performance batteries or "synthetic" work. Here the time-sharing demands of simultaneously monitored and performed tasks approximate mission conditions. However, predictive validity is still an issue. Greater predictive validity can be obtained through the use of partial and full-scale simulation; but it is often difficult to isolate individual work units or to identify the particular environmental stressors of interest.
We cannot rely on any single approach to supply all of the information necessary to plan performance requirements for future missions. What appears to be most needed at this time is a more integrated approach using various techniques, along with a greater emphasis on data from actual missions. Using data from missions, researchers could begin to unravel the many complex factors involved in the definition and measurement of performance in space. A kind of performance laboratory in space could provide the opportunity to explore the complexities of performance unhindered by the requirements of mission operations. Findings from such studies could be validated by the selected measuring of performance on mission tasks. Recent interest in and support for the concept of such a Space Station has been encouraging. It is hoped that continued efforts can be made to bring this objective to fruition.
Beyond the basic issues of defining and measuring performance, we are faced with the task of developing a better understanding of the many factors of work in space that may influence performance  levels. Weightlessness, space sickness, biomedical changes, and protective clothing, all may operate to reduce the work capacity of crews in space. Continuing research is needed to consider how these individual elements may affect work capacity, particularly under the extended exposures of a long-term mission. Mission goals, the amount and sequencing of daily work schedules, and the type and amount of preflight training must all be geared to the capabilities of the operator in space.
It may be necessary to balance work schedules in space to minimize the problems of overloading/underloading and the resulting condition of fatigue that can adversely affect performance. Such approaches as crew-selected work schedules or rotational work schedules should be examined. Also, the degree of overtraining required prior to flight for all tasks regardless of their relative importance must be questioned. If all work is overlearned, the problem of boredom may be exacerbated. The prospect of initiating a program of on-board learning should be considered. In this way, at least some aspects of mission-required work can continue to be novel and stimulating.
Desynchronosis can adversely affect performance. Precautions must be taken to ensure that controllable conditions of space and the requirements of work schedules do not jeopardize the synchronization of circadian rhythms. Continuing research is needed to determine the significance of various zeitgebers or cues for particular rhythms and their adapatability to space. It is important, too, that planners recognize that performance itself is subject to circadian cycles. Research could help identify patterns of performance rhythms and determine how work schedules could be arranged to maximize productivity.
Coincident with the study of work schedules and desynchronosis, the importance of sleep must be considered. Changes in either the quality or duration of sleep can have a profound impact on performance. Questions remain as to how sleep may be affected by long-term confinement and isolation, by the restrictions of the space vehicle, and by lack of gravity. One ground-based research avenue might involve the use of air- or water-body support systems to study the effects of reduced body pressure on sleep patterns.
Sustained, high-quality performance of crews in space is necessary for the safety of the crews and the successful completion of  mission tasks. As we look toward future extended flights, we can anticipate a need to continuously refine our understanding of the complexities of work in the space environment.