Question 1:

A case-control study was performed to determine whether head injury was associated with an increased risk of brain tumors in children. 200 cases with brain cancer were identified from the state cancer registry and 200 controls were recruited from the same neighborhoods where the cases lived. The mothers of the children completed a questionnaire that asked them to describe their child's past history of head injury. The investigators found that the mothers of the children with brain tumors reported a past head injury for 70 of the cases while a past history of head injury was reported in 30 of the controls. What type of bias was likely to have influenced the findings of this study, and why? What can be done to minimize this type of bias?

Question 2:

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.

Question 3:

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:

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.

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.