The decision making processes of children in a probabilistic environment were studied within the context of the theory of signal detection (TSD). The relationship between the age of the child and his ability to revise decision criteria was examined, as well as the appropriateness of TSD measures and methodology with children. It was hypothesized that the decision criteria of older children would be more optimal than the younger children. Twenty-four females with normal I.Q.'s at three age levels (kindergarten, elementary school, and college) were administered a visual signal detection task. The prior probability of the signal was manipulated for half the subjects, and the value of the signal was manipulated for the other half. The results did not support the experimental hypothesis, since there was no age difference in decision criteria change. In addition, the results indicated that children as young as 4 to 5 years of age did operate according to TSD given the conditions of the model. Therefore, the TSD paradigm is appropriate for studying probabilistic thinking in children. (Author/GDC)