According to a study done at Kaiser Permanente in Walnut Creek, California, users of oral contraceptives have a higher rate of cervical cancer than non-

users, even after adjusting for age, education, and marital status. Investigators concluded that the pill causes cervical cancer. 21

(a) Is this a controlled experiment or an observational study?

(b) Why did the investigators adjust for age? education? marital status?

(c) Women using the pill were likely to differ from non-users on another factor which affects the risk of cervical cancer. What factor is that?

(d) Were the conclusions of the study justified by the data? Answer yes or no, and explain briefly. plo47