PU570 Unit 6 Causation Debate Reflection Example

I Was Assigned to Argue Causation and Left Doubting Both Sides: A Unit 6 Seminar Reflection on Genital Talc Use and Ovarian Cancer, Where the Case-Control Studies and the Cohorts Disagree for Reasons Neither Team Would Name

[Student Name]

Purdue University Global

PU570: Chronic Disease Epidemiology

Unit 6 Seminar Reflection

[Instructor Name]

[Date]

The seminar and its two teams are the course's own; the studies, their figures and the disagreement between designs are real.

What I Was Given to Argue

That genital powder use causes ovarian cancer. My team's material was the pooled case-control evidence: across eight studies with 8,525 cases and 9,859 controls, ever use was associated with about a 24 percent higher odds of ovarian cancer, with the association appearing for several histologic types (Terry et al., 2013). My opening was straightforward and the figures were real.

What the Other Team Had

One analysis, and it was enough to spend the hour on. Pooling four prospective cohorts with more than 250,000 women, ever use carried a hazard ratio of just 1.08, an interval that still contained one, and no clear pattern by duration or frequency (O'Brien et al., 2020). The cohorts measured exposure before diagnosis; my case-control studies asked women after it.

Where the Hour Went

The useful part was not either argument but the question of why the designs differ. Three explanations were on the table. Recall. A woman with ovarian cancer who has read about talc searches her memory differently from a control. This explains part of the case-control excess and the direction is predictable. Power. Ovarian cancer is uncommon, so even 250,000 women across four cohorts produce a confidence interval wide enough to contain a real small effect. The cohort result is consistent with a hazard ratio of 1.0 and also with 1.17, which is close to the case-control figure. Exposure definition. Ever use is a weak variable in both designs. The cohorts asked about use at enrollment and not before it, so women who used powder for decades and stopped are mixed with recent users. Neither team had the evidence it was claiming. My side had a real association that recall bias could produce, and their side had a null result that was underpowered to exclude the effect I was arguing for.

What I Said at the End, Against My Own Brief

When the facilitator asked me to summarize, I said that the honest position is that a small effect cannot be ruled out and has not been demonstrated, and that the case-control and cohort estimates are closer to each other than either team's rhetoric suggested. One teammate thought this was a concession. I think it was the finding of the seminar.

What I Had Not Appreciated Before the Hour

That a null result and a negative result are different things, and that most published commentary on this question treats the second cohort analysis as the latter. A confidence interval from 0.99 to 1.17 does not establish absence. It bounds the effect, which is a real contribution and not the same as a refutation. My team had made the opposite error, treating a pooled odds ratio of 1.24 from recall-based studies as a measurement.

How Hill's Considerations Would Read Here

Applying the 1965 considerations to this question is sobering (Hill, 1965). Strength is weak in both designs. Consistency is mixed by design type, which is the most informative thing about the evidence. A biological gradient is absent in the cohorts and inconsistent in the case-control pooling, and gradient is usually the consideration that separates a causal association from a biased one. Plausibility exists, with migration of particles to the upper genital tract and an inflammatory mechanism, but plausibility is cheap. The consideration I would weigh most is the gradient, and it is the one the evidence most clearly fails to supply.

Two Rules I Am Taking Back to My Own Analyses

Two things. When a case-control estimate and a cohort estimate disagree, I will write out the three candidate explanations before choosing one, since my instinct in the seminar was to reach for recall bias because it favored neither of my positions. And I will stop reporting an interval that includes one as a null finding, and report its upper bound instead, because the upper bound is what a reader needs in order to know what the study ruled out.

What I Still Cannot Resolve

Whether this question is answerable with observational data at all. The exposure has declined, litigation has made recall unreliable in both directions, and no trial will be done. A seminar cannot settle that, and I suspect the field's honest answer is that the effect, if it exists, is small enough that the designs available cannot separate it from their own biases, which is an uncomfortable conclusion for a course that teaches those designs.

References

Hill, A. B. (1965). The environment and disease: Association or causation? Proceedings of the Royal Society of Medicine, 58(5), 295-300. https://doi.org/10.1177/003591576505800503

O'Brien, K. M., Tworoger, S. S., Harris, H. R., Anderson, G. L., Weinberg, C. R., Trabert, B., Kaunitz, A. M., D'Aloisio, A. A., Sandler, D. P., & Wentzensen, N. (2020). Association of powder use in the genital area with risk of ovarian cancer. JAMA, 323(1), 49-59. https://doi.org/10.1001/jama.2019.20079

Terry, K. L., Karageorgi, S., Shvetsov, Y. B., Merritt, M. A., Lurie, G., Thompson, P. J., Carney, M. E., Weber, R. P., Akushevich, L., Lo-Ciganic, W. H., Cushing-Haugen, K., Sieh, W., Moysich, K., Doherty, J. A., Nagle, C. M., Berchuck, A., Pearce, C. L., Pike, M., Ness, R. B., ... Goodman, M. T. (2013). Genital powder use and risk of ovarian cancer: A pooled analysis of 8,525 cases and 9,859 controls. Cancer Prevention Research, 6(8), 811-821. https://doi.org/10.1158/1940-6207.CAPR-13-0037