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This note analyzes several recent experimental and theoretical reports on vigilance that use the theory of signal detectability (TSD). The psychological interpretation of the TSD measure of criterion, β, as an index of conservativeness during a vigil does not appear to be valid. As computed, β is probably an artifact due to pooling observations made under different conditions of attentiveness during a long vigil. The basic problems of vigilance research remain: to determine the condiitons that affect attentiveness in signal detection tasks.

Vigilance as a human factors area is concerned with the fact that man is much less capable as a detector of signals under operational conditions than would be indicated by laboratory measures of his sensory thresholds. The area, which is obviously important for the analysis of man's visual capabilities in the operation of manned space systems, is re-examined with the help of a theoretical model that introduces a decision–theory approach to the observing response phase of the vigilance task. After a critical review of the vigilance literature, examples are presented of the application of this approach to the solution of human factors problems of the sort that might arise in manned space missions. The model is also used to suggest the kind of research that would make it easier to predict field monitoring performance from laboratory experiments.

Observers detected many more of a fixed number of signals when these were among stimuli presented at 5 per minute than when these were among stimuli presented at 30 or 60 per minute. The effect, which is associated with either the signal probability or the nonsignal stimulus density, is analyzed with conventional measures and with measures from the theory of signal detectability (TSD). The TSD measures were used to define several possible modes of observing, and the model of vigilance based on decisions about observing could then be related to decision processes in detection performance as considered by TSD. If a single measure of the probability of alert observing is required, the best one is the percentage of detections of the readily detectable signal of the vigilance task. However, the TSD analysis suggested various different "mixes" of modes of observing for the subgroups in this experiment, and these mixes could be specified with the the help of heuristic models relating performance measures to the probability of observing.