A cross-sectional survey in a rural Indian district investigates the association between well water consumption and fluorosis. The crude prevalence ratio (PR) for fluorosis among well water users is 2.8 compared to non-users. However, when the data are stratified by age group, the PR in children (5–12 years) is 2.5 and in adolescents/adults (≥13 years) is 2.6. The crude PR of 2.8 lies outside the range of stratum-specific PRs. Which investigation should be performed to quantify the magnitude of confounding by age?