Broadbent and Gregory Revisited: Vigilance and Statistical Decision

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Abstract
By rigorous test, Broadbent and Gregory (1963) showed that the behavior of subjects in a vigilance task could be adequately described by the equal variance model of signal detection theory (SDT). Since then the model has been widely applied in the analysis and interpretation of numerous vigilance studies, but its goodness of fit to the data has been met by mere assumption. The present study attempted to redress the balance by testing the assumption in respect of some 200 individual ROCs obtained from four vigilance experiments. Although the analysis indicated that about half of the individual ROC functions were of the curved form predicted by the equal-variance SDT model and hence support the view that vigilance performance can be interpreted in terms of this theoretical model of decision-making, a substantial proportion (30%) of the individual functions were of a particular form whose presence is difficult to account for, if handled within the SDT framework. Other models which seem more appropriate are suggested and the implications discussed.