Suppose that a cohort study of body mass index and coronary heart disease (CHD) was undertaken. Participants were classified as having either a high or normal body mass index. Because age is associated with both body mass index and risk of CHD, age was considered a potential confounder and the age of each participant was recorded. The following data were gathered from the study participants: Normal Body Mass Index High Body Mass Index
DISEASE STATUS AGE
Young Old DISEASE STATUS AGE
Young Old
Developed CHD 18 65 Developed CHD 20 200
Did Not Develop CHD 6982 2935 Did Not Develop CHD 3980 5800
Total 7000 3000 Total 4000 6000

A. Set up the two by two to evaluate the crude association between body mass index and CHD and calculate the crude risk ratio.
B. Set up the two by two tables to evaluate the relationship between body mass index and CHD stratified by age. Calculate the risk ratios separately for the young and old participants.
C. Is age a confounder in this study?
D. Is age an effect modifier in this study?
E. If age is an effect modifier, describe how age affects the ratio measure of comparison between body mass index and CHD risk