This study used a rule-analytic technique to investigate the role of event covariation in causal judgment. Junior high school and college subjects were shown information about the co-occurrences of two potentially related events and were asked to make either causal or covariation judgments about the two events. Subjects often failed to identify covariates as causes or identified as causes events which were either unrelated or related in the opposite direction to the event to be explained. Rule analyses indicated that use of mathematically flawed strategies resulted in erroneous covariation and causal judgments. Comparisons between the junior high and college samples showed parallel improvement with increasing age for the two judgments. Strategy analyses of the covariation and causal judgments showed that males defined causes and covariates by similar rules, but that females used different rules to make the two judgments.