Wednesday, February 20, 2008

How Science Works (3): Causation and Association

In the late 1990s researchers discovered that virtually all the women with cervical cancer in the world had something in common: they were all infected with certain types of human papillomavirus (HPV). They also knew that many women with cervical cancer were also infected with genital herpes simplex virus (HSV). Both genital HSV and HPV infections were associated with cervical cancer, but were they the actual cause of the disease?

Association and causation are not the same. While an association is an identifiable relationship between an exposure (HSV or HPV infection) and a disease (cervical cancer), causation implies that there is a true mechanism that leads from exposure to disease.

Later, scientists found that, in fact, cervical infection with certain types of HPV is necessary for a woman to develop cancer of the cervix: all of the cervical cancers and pre-cancers contain one of these viruses. Furthermore, an HPV vaccine protects against both infection and the cancer caused by the HPV types in the vaccine (but not against most other HPV types). This evidence suggested that there was more than an association; HPV was indeed the cause of cervical cancer.

In general, a disease is caused by an event or condition that precedes the disease and without which the disease would not have occurred. And most diseases have more than a single cause.

It is important that as medical writers we communicate clearly the distinction between an association and a cause-and-effect relationship. For instance, one study may find an association between eating chocolate and depression, but it doesn't mean that chocolate causes depression--it may simply be that depressed people in the study ate more chocolate than the control group.

What would happen if we don't communicate clearly that distinction? We may see a decline in chocolate consumption in the world (hard to imagine, I know, but bear with me for the example's sake).

I think the issue of causation is fascinating, so I'll keep writing about it in future posts. Happy medical writing!

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