A Prospective Cohort Study Was Conducted to Assess the Effects

A prospective cohort study was conducted to assess the effects of oral contraceptive (OC) use on the incidence of breast cancer.  A crude analysis of the study suggests an association between OC use and breast cancer (relative risk [RR] = 1.40, 95% confidence interval [95%CI]: 1.01, 1.96) .  Further analysis shows that in women with a family history of breast cancer, OC use increases the risk of breast cancer (RR = 3.33, 95%CI: 2.51, 4.41) .  In women without a family history of breast cancer, a significantly smaller effect is observed (RR = 1.27, 95%CI: 1.01, 1.60) .  The results of this study are best explained by which of the following?

A) Confounding bias
B) Effect modification
C) Latency period
D) Observer bias
E) Selection bias

Correct Answer:

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