Developmental patterns in one mode of formal reasoning (probabilistic reasoning) were examined in this research study, specifically: (1) to determine if developmental patterns of probabilistic reasoning could be confirmed using a large, diverse sample of subjects, and if they could be similar despite variations in the complexity of the reasoning required to solve a problem; (2) to determine if variations in problem context led to variations in response patterns; and (3) to determine if developmental patterns of probabilistic reasoning were similar for subjects of similar reasoning ability regardless of education level. Data from 2282 subjects in grades 6-13 were collected using Tobin and Capie's Test of Logical Thinking (TOLT), form A (N=1627) and form B (N=655). For each of the four items, the proportion of students obtaining correct responses was linearly related to their formal reasoning ability and a measure of the TOLT. Two common approaches were identified for individuals attempting to use probabilistic reasoning to solve problems. (DS)