Knowing what we’re doing: correlation, causation, or randomness

September 21, 2011  |  General

Medicine is particularly prone to be trapped by the well-known fallacy with the fancy Latin label of post hoc, ergo propter hoc — or, after this, therefore because of this. This is because, when a patient comes to us with a particular problem, physicians generally do things, like prescribing medicines and do procedures. We intend for what we do to help the situation, but often there is no way to tell if it has. The patient may improve; or the patient may not. What follows may be related to our treatment, or it may not be.

Even if there is good scientific evidence that a treatment works, that evidence generally relates to populations of people, not an individual. And individuals are, well, individual. A person is not a statistic. So if a treatment is a really, really good one, for example one that helps 90% of the time, 10% of the time it won’t help. It might even make the condition worse for some individuals.

Anyone who has watched late-night cable television has seen countless examples of this logical trap, in the form of personal testimonials from people who had this or that problem, took the pill or bought the product, and the problem went away. The fallacy, of course, is that the two events may be entirely unrelated, just as the fact I may drink coffee every morning before the sun comes up does not cause the heavens to move in that way. I’ve written about this in a little more detail here.

This uncertainty can drive a precise mind a little crazy. But for me it’s one of the things that makes practicing medicine fascinating. Medicine isn’t a science: it’s a mishmash of science, near-science, educated guessing, and blind luck. Sort of like life.

(Thanks to MacAllister Stone for the link to the pithy illustration of this dilemma.)

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