Question

A hypothetical case-control study found that regular coffee drinkers were 1.8 times more likely to report a positive sense of well-being than non-coffee drinkers (OR = 1.8; 95% CI = 1.3-2.4). When the results were stratified by gender, the odds ratio for women was 1.1 (95% CI = 0.9-1.3) and the odds ratio for men was 0.9 (95% CI = 0.7-1.1)

. Which of the following statements best describes the role of gender in the association of coffee drinking and sense of well-being? a. Gender is both a confounder and an effect modifier of the association between coffee drinking and sense of well-being. b. Gender is a confounder of the association between coffee drinking and sense of well-being. c. Gender is an effect modifier of the association between coffee drinking and sense of well-being. d. Gender is neither a confounder nor an effect modifier of the association between coffee drinking and sense of well-being.

Answer 1

If in a sub-group (based on suspected confounder/effect modifier) analysis, the original association between the exposure and outcome doesn’t hold up in BOTH sub-groups the factor is a confounder. But if the original association holds up in one group but not in another, then the factor is an effect modifier.

Since, confidence interval of odds rato for men and women contains the value 1, the role of gender in the association of coffee drinking and sense of well-being is not significant. But the overall confidence interval of odds rato does not contain the value 1, and thus the the association of coffee drinking and sense of well-being is significant.

So, b. Gender is a confounder of the association between coffee drinking and sense of well-being.