A cohort study was undertaken to examine the association between high lipid level and coronary heart disease. Participants were classified as having either a high lipid level (exposed) or a low or normal lipid level (unexposed). Because age associated with both lipid level and risk of heart disease, age was considered a potential confounder, and the age of each subject was recorded. 

The following data describes the study participants:

Developed CHD.       No CHD

Exposed (high lipid level)

young    20.         3980

Old.      200.         5800

Unexposed (Low or normal lipid level)

Young.    18.           6982

old.      65            2935

A. Calculate the appropriate crude ratio measure of association combining the data for young and old individuals. 

B. For the same cohort study, perform a stratified analysis and calculate the appropriate stratum- specific ratio measures of association. What are they?

C. Do the data provide evidence of effect measure modification on the ratio scale? Justify your answer. 

1. Which Two study designs provide the best evidence to support Hill's causal guideline on temporality? Be as specific as possible?
2. Describe in 1-2 sentences Hill's responses for suggesting that strong associations are more likely to be causal than weak assoications?