Bias and Confounding

Regarding confounding variables in epidemiological studies, all of the following are methods to control for confounding EXCEPT:

Which of the following is a characteristic feature that distinguishes confounding from bias in epidemiological studies?

In a cohort study investigating the association between oral contraceptive use and venous thromboembolism, smoking status is found to be associated with both the exposure and outcome. What type of epidemiological error is smoking status in this scenario?

In a case-control study investigating the association between oral contraceptive use and venous thromboembolism, researchers found that women with VTE were significantly more likely to report a family history of thrombophilia than controls. However, when the analysis was stratified by thrombophilia status, the association between OCP use and VTE disappeared in both strata. Which type of bias or confounding is most likely responsible for this finding?

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?