Tag Archives: Medicine

The Post Hoc Fallacy

Correlation is not causation.

It seems a simple statement when you look at it. Just because night follows day does not mean that day causes night. However, it’s clear that people fall prey to this fallacy all the time. It’s what’s behind, for example, the superstitious rituals of baseball pitchers.

A far less trite example is modern medicine. You have a headache. You take a pill. Your headache goes away. Did it go away because of the pill you took? Maybe it would have gone away on its own.  How do you know?

Teasing out causation from mere correlation in cases like that, with potentially dozens of unknown and uncontrolled variables, is notoriously difficult. The entire industry of complimentary and alternative medicine banks on the confusion.

I was thinking about all this the other day when I was testing a tool that takes mailed orders for prescription drugs, digitizes the data, and then adds it all to a central database. I was focusing specifically on the patient address information at the time, so the rest of the orders, like payment information, was fairly simple–meaning all my test orders were expected to get assigned a payment type of “invoice”, which they did. So in the course of my address testing I “passed” the test case for the invoice payment type.

It wasn’t until later that I realized I had committed the fallacy Post hoc ergo propter hoc (“After this, therefore because of this”), just like the person who attributes the disappearance of their headache to the sugar pill they’ve just taken. I discovered that all orders were getting a payment type of “Invoice”, regardless of whether they had checks or credit card information attached.

Inadvertently, I had succumbed to confirmation bias. I forgot, momentarily, that proper testing always involves attempting the falsification of claims, not their verification.

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