Note: This is meant to be an accessible introduction to causal inference. Comments appreciated.

Let's say you buy a basil plant and put it on the counter in your kitchen. Unfortunately, it dies in a week.

So the next week you buy another basil plant and feed it a special powder, Vitality Plus. This second plant lives. Does that mean Vitality Plus worked?

Not necessarily! Maybe the second week was a lot sunnier, you were better about watering, or you didn’t grab a few leaves for a pasta. In other words, it wasn’t a controlled experiment. If some other variable like sun, water, or pasta is driving the results you’re seeing, your study is confounded, and you’ve fallen prey to a core issue in science.

When someone says “correlation is not causation,” they’re usually talking about confounding. Here are some examples:

• A 2019 study found that student test [...]

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