Tuesday, October 4, 2011

Spot the Problem

Here's some excerpts from a recent NYT article.  What's wrong?  You should probably at least suspect it from the very first sentence.
The most popular contraceptive for women in eastern and southern Africa, a hormone shot given every three months, appears to double the risk the women will become infected with H.I.V., according to a large study published Monday.  
...In each couple, either the man or the woman was already infected with H.I.V. Researchers followed most couples for two years, had them report their contraception methods, and tracked whether the uninfected partner contracted H.I.V. from the infected partner, said Dr. Jared Baeten, an author and an epidemiologist and infectious disease specialist.
...The study found that women using hormonal contraception became infected at a rate of 6.61 per 100 person-years, compared with 3.78 for those not using that method. Transmission of H.I.V. to men occurred at a rate of 2.61 per 100 person-years for women using hormonal contraception compared with 1.51 for those who did not.
First person to answer correctly gets one economonomadollar ($1$).  Currently the $$ is not accepted across the US, but every fiat currency has to start somewhere. If nothing else, the American dollar can be used to pay your taxes, so to give the $$ some backing, let's say that I'm currently selling my answer to any question you might have, for $1$ per answer.  [As a disclaimer, please note that I said my answer, not the answer.  This ensures that I can always make good!]


  1. Selection?

    If my first read was correct (NYT isn't letting me read it again b/c I don't subscribe), the study compares women who voluntarily signed up for the contraceptive to women who used no such contraceptive (who may have been abstaining). Makes sense: An experiment would be tough to justify morally.

    As far as I can tell, they don't do anything to distinguish among three competing explanations (a) the physical effects of the drug, (b) the behavioral effects of the drug and (c) the HIV-risk type selection into using the drug. It may be that injections correlate positively with HIV risk due to all three of these channels, but without additional controls (for example, choosing a comparison group who used a different form of contraception), it is hard to tell.

    Then again, they might have done more work on this issue in the study and I didn't catch it when I skimmed the article.

  2. That's good, T-Bone, and I award you $1$. But even if there is no selection, I'm worried about a specific and likely behavioral effect of this drug.

    There is still $1$ for someone (else) who tells me: even if we had randomly assigned some women to get the hormone shot, why might the results of this study not be surprising?

  3. whether or not you use a form of contraception affects sexual behavior which affects transmission rates. Depends if "not using that method" means not using that method of contraception but using contraception or anyone not using that method of contraception (including people who used no contraception).

  4. $1$ for TAllen, that's good enough for me. In particular, we might imagine that people who are taking this contraceptive will *have more sex* since the contraceptive makes it less costly overall. The weird thing is that they measure "transmission rate" per year instead of per sexual encounter.

    In retrospect I might have included the second line of the article:
    "And when it is used by H.I.V.-positive women, their male partners are twice as likely to become infected than if the women had used no contraception."