In a hypothetical prospective cohort study, investigators compared the incidence of thromboembolism (TE) in 10,000 women on oral contraceptives (OC) and 10,000 women not taking OC. Results are shown in Table 3.

Table 3: Results of Study

	Thromboembolism	No thromboembolism	Total
Oral contraceptive use	20	9,980	10,000
No oral contraceptive use	10	9,990	10,000
Total	30	19,970	20,000

1a Compute the risk ratio comparing thromboembolism risk between those exposed to oral contraceptives versus those unexposed to oral contraceptives. Provide an interpretation of this risk ratio. (4 points)

Suppose there were substantial losses to follow up in both groups as the study was a 20- year cohort study. Amongst those in the oral contraceptive group and at greatest risk of thromboembolism, 10% were lost to follow-up. Amongst all other study participants, 20% were lost to follow-up.

1b. Are you concerned about bias in this study? If so, indicate what kind of bias and indicate the impact on the risk ratio calculated in Question 1. If not, indicate why not.(4 points)