Third-Source Causation: An Alternative Explanation for the Check Mark Pattern
Abstract
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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Aug 01, 2006
- Accession Number
- ADA455555
Entities
People
- William G. Jackson