duces pain.)
- **Selection:** Differences in the types of people who are treated. (People who take Advil are in more pain.)

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The central challenge of causal inference is to disentangle treatment from selection. How?
- Since this formula is about *potential* outcomes, we can never prove it.
- We always need to bring in (well-grounded) assumptions.

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**The idea:** find a situation where we can argue that selection bias is negligible.
- This is called a **research design** or **identification strategy.*