Male circumcision — a case of selection bias

10 Mar, 2015 at 17:01 | Posted in Statistics & Econometrics | 1 Comment

Take a look at a map of Africa showing male circumcision rates, and impose on that data on HIV/AIDS prevalence. There is a very close correspondence between the two, with the exceptions being cities with large numbers of recent uncircumcised male migrants. One might therefore conclude that male circumcision reduces the chances of contracting HIV/AIDS, and indeed there are medical reasons to believe this may be so. But maybe some third, underlying variable, explains both circumcision and HIV/AIDS prevalence. meme2That is, those who select to get circumcised have special characteristics which make them less likely to contract HIV/AIDS, so a comparison of HIV/AIDS rates between circumcised and uncircumcised men will give a biased estimate of the impact of circumcision on HIV/AIDS prevalence. There is such a factor, it is being Muslim. Muslim men are circumcised and less likely to engage in risky sexual behaviour exposing themselves to HIV/AIDS, partly as they do not drink alcohol. Again we are not comparing like with like: circumcised men have different characteristics compared to uncircumcised men, and these characteristics affect the outcome of interest.

Howard White

1 Comment

  1. Obviously a random trial would be best, but would data from the US help to solve the problem,too? In the US circumcision is widespread also among non-religious people.


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