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Simplest example of Simpson's paradox

I had read many times about Simpson's paradox but it never quite clicked for me. I saw many examples, but I couldn't quite get what was really going on. And I could never remember the numbers in the examples, so I couldn't ponder it while waiting for the bus or whatever.

Last month I sat down and thought about it, with the idea of coming up with the simplest and most memorable possible example. Here it is.

• Imagine there is a disease that kills 50% of the people who get it.

• There is a pill you can take if you get the disease. Of the people who take the pills, 80% die.

• It looks like the pills are killing people. But they aren't, they are helpful.

This is Simpson's paradox.

• What is really happening is that half the people with the disease have mild cases and half have severe cases. A patient with a mild case will get better on their own. But everyone with a severe case dies unless they receive treatment.

• People with mild cases don't bother to take the pills, because they are going to get better anyway.

• Only people with severe cases take the pills. 80% of them die, but without the pills they all would have died.

Now I understand Simpson's paradox.

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