1. A cohort study was undertaken to examine the association between high lipid level and coronary heart disease (CHD). Participants were classified as having either a high lipid level (exposed) or a low or normal lipid level (unexposed). Because age is associated with both lipid level and risk of heart disease, age was considered a potential confounder or effect modifier and the age of each subject was recorded. The following data describes the study participants: Overall, there were 11,000 young participants and 9,000 old participants. Of the 4,000 young participants with high lipid levels, 20 of them developed CHD. Of the 6,000 old participants with high lipid levels, 200 of them developed CHD. In the unexposed, 18 young and 65 old participants developed CHD.
• Construct the appropriate two by two tables using the data given above. Be sure to label the cells and margins.
• Calculate the appropriate crude ratio measure of association combining the data for young and old individuals.
• Now, perform a stratified analysis and calculate the appropriate stratum-specific ratio measures of association. What are they?
• Do the data provide evidence of effect measure modification on the ratio scale? Justify your answer.

A study followed 900,000 US adults from 1992 to 2008. At baseline, all participants were screened and determined to be cancer free and their body mass index (BMI) was calculated. Body mass index is a measure of obesity that is calculated using a person’s height and weight. Subjects were separated into the following groups according to their BMI: (a) normal weight, (b) slightly overweight, (c) moderately overweight and (d) greatly overweight. 57,145 deaths from cancer occurred in the population during the follow-up period. . The following results were seen for men and women when the heaviest members of the cohort (greatly overweight) were compared to those with normal weight:
Men: Risk ratio of cancer death = 1.5, 95% confidence interval = 1.1–2.1
Women: Risk ratio of cancer death = 1.6, 95% confidence interval = 1.4–1.9
• State in words your interpretation of the risk ratio given for the men.
• State in words your interpretation of the risk ratio given for the women.
• Are these results confounded by gender?
• The authors stated that they controlled for confounding many risk factors using a multivariate analysis. State an alternative method that the authors could have used to control for confounding in the design or analysis. In addition, name two confounding variables that you think should be controlled using this method.

The association between cellular telephone use and the risk of brain cancer was investigated in a case-control study. The study included 475 cases and 400 controls and the following results were seen:
Cases Controls
Cellular Phone User
Yes 270 200
No 205 200
Total 475 400

• Calculate the odds ratio based on these data.
• The p value for this odds ratio is 0.06. State your interpretation of this p-value.
Gender was considered a potential confounder and effect measure modifier in this study. The data were stratified into males and females in order to assess these issues.
Males, Females
Cases, Controls, Cases, Controls
Cellular Phone User
Yes 242 150, Yes 28 50
No 100 50, No 105 150
• Calculate the stratum-specific odds ratios.
• Is gender a confounder in this study? Briefly justify your answer.
• Is gender an effect measure modifier in this study? Briefly justify your answer.

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