The percentage of detection of 37 studies of vigilance, using simple signals, was found to depend primarily on the initial or pretest detection level, the nature of the signal, i.e., whether it is a dynamic signal (requires movement or change of state of the eye) or static, and the duration of the watch. Using the data of these studies, a watchkeeping decremental function was developed. It was shown that the function can be used to estimate a tradeoff between time of watch and the initial percentage of detection. The loss of detection associated with static signals was found to be relatively small. It is suggested that it is this small loss which represents a vigilance process. The greater decrements associated with dynamic stimuli are assumed to be due to an additional ocular demand.