The effects of specificity of probability information on human performance were investigated in two experiments differing only in task difficulty. Subjects decided whether a stimulus was a #x2018;friend#x2019; or #x2018;foe#x2019;. Four signal detection-aids presented the probability of a #x2018;foe#x2019; according to differing levels of specificity: (a) two-categories, (b) four-categories, (c) six-categories, and (d) eight-categories. In the fifth condition, the control, no signal detection-aid was present. Hick's law applied to the unsure and correct response times for the four (2#xB7;0 bit), six (2#xB7;5 bit), and eight (3#xB7;0 bit) category aids for both experiments. An inverted U-shaped function was hypothesized for sensitivity (da) as a function of aid specificity. In the easier task the eight-category aid and the two-category aid group sensitivity was not significantly different from the other aids. However, the eight-category aid group had greater confidence than the other groups. For the harder task there was no reduced sensitivity for subjects using the eight-category aid, but the two-category aid had reduced sensitivity. In both experiments the mean criteria (B#x2032;) for the aided conditions were too liberal when the aid indicated a low probability (M= 0#xB7;2) for a #x2018;foe#x2019;.
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