(A) Ziel, H K., Finkle W D. Increased Risk of Endometrial Carcinoma Among Users of Conjugated

Ashley Viola

Assignment 1 - Selection Bias

(a) Ziel, H K., Finkle W D. Increased risk of endometrial carcinoma among users of conjugated estrogen. NEJM 299(23): 1167-1170, 1975 (b) R. D. Daniels, T. D. Taulbee and P. Chen. Radiation exposure assessment for portsmouth naval shipyard health studies. Radiation Protection Dosimetry 2004 111(2):139-150.

(a) Here selection bias results from selection of estrogen users for detection of disease because of vaginal bleeding that is frequently induced by estrogen. Essentially they chose patients with vaginal bleeding which indicate their use of estrogen. Controls were matched on date of birth, residence, health care plan, and the potential for the development of endometrial cancer by the control subject’s having an intact uterus. Conjugated estrogen use was 57% of the 94 patients with endometrial carcinoma and 15% for controls. The corresponding point estimate of the risk ratio was 7.6. The risk ratio increased with duration of exposure from 5.6 for 1-4.9 years of exposure to 13.9 for 7+ years. These results suggest that conjugated estrogen have an etiologic role in endometrial carcinoma.

Women who were receiving the estrogen were probably more likely to seek medical attention and thus have a greater chance of detecting a problem.

(b) The healthy worker effect is another form of selection bias. The healthy worker effect occurs in this study because the general population, which consists of both healthy and ill people, is selected for comparison to a relatively healthy working population. Radiation- exposed workers in this study experienced a lower mortality rate than the general population. This was likely because the radiation-exposed workers were required to undergo special physical examinations to be eligible for work and those that did not pass examination were removed from the program.

(a) A way to eliminate this kind of selection bias in this case could be to select controls that are also likely to develop cancer due to a tumor and undergo routine screenings for an illness such as cysts/polyps or other relevant diseases.

(b) A way to reduce the healthy worker effect in this study could be to sample from those who work in the same shipyard, but do a different task that does not expose them to or have limited exposure to radiation. This would allow the sampling of individuals that are all working and work in similar environments, but have a different level of exposure. Another method to eliminate the bias is to attempt to locate another shipyard where the workers are not or have a limited exposure to radiation and use this population as the comparison group. Both of these suggestions allow you to utilize a working population thus comparing two populations with a similar health status.