Figure 5. Elevation’s association with lung cancer is consistent across county strata.

Stratification analyses provide no evidence for a smoking-elevation interaction or state-based confounding. (A) Lung cancer incidence (age-adjusted per 100,000) and elevation (km) are plotted for counties stratified into terciles by smoking prevalence. Counties were shaded by their regression weight, and a bivariate regression was fit for each stratum. The non-overlapping confidence bands (95%) illustrate the strong effect of smoking on lung cancer, while the approximately parallel slopes demonstrate the lack of an observable smoking-elevation interaction. (B) Lung cancer was regressed against elevation and smoking for each state. State-specific elevation coefficients are plotted with 95% confidence intervals. Ten of eleven states displayed negative coefficients. State-specific elevation coefficients were meta-analyzed. The resulting 99% confidence interval overlaps the interval from the optimal best subset model shown in blue. The number of counties within each state analysis is indicated in parentheses.