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Risk Factors for Ectopic Pregnancy

Noel S. Weiss, MD, DrPH; Janet R. Daling, PhD
JAMA. 1988;260(11):1553-1554. doi:10.1001/jama.1988.03410110061019.
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To the Editor.—  In the report of their case-control study that appeared in the March 25 issue of THE JOURNAL, Marchbanks et al1 estimated the risk of ectopic pregnancy in current users of intrauterine devices (IUDs) to be 13.7 times that of nonusers. However, we believe that because of the way the authors defined their control group that this value seriously overstates the true excess risk associated with the use of an IUD.Ideally, controls in a case-control study should be all or a sample of that population at risk for the disease from which the cases arose. For practical reasons, often this ideal is not met, and it is necessary to choose as controls persons who have conditions or illnesses that are different than the one under investigation. This choice will produce a valid result only to the extent that the controls selected have a frequency of exposure


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