Third-Source Causation: An Alternative Explanation for the Check Mark Pattern

The Air Force Health Study AFHS is a 20-year longitudinal study examining the health, mortality and reproductive outcomes of the Air Force Operation Ranch Hand veterans responsible for handling and aerially spraying herbicides including Agent Orange, several of which were contaminated with 2,3 ,7,8-tetrachlorodibenzo-p-dioxin TCDD, in Vietnam from 1962 to 1971. Several statistical analyses performed in the conduct of the AHFS have produced results that exhibit apparently paradoxical findings. These findings are characterized by a significant between-group difference Ranch Hand mean greater than Comparison on the independent variable X serum TCDD, significant within-group correlations having the same sign between X and a dependent variable Y, but no overall between-group difference between the two groups on Y. Various explanations put forward to account for this phenomenon include direct causation, reverse causation, misclassification bias, and differential TCDD elimination. We investigated the possibility that a different explanation, causation of X and Y by a common third source unrelated to group membership, could account for the pattern.