Mathematics
Causal Inference Using Instrumental Variables
Quick fact
Instrumental variables can uncover causal effects even when we cannot measure or control for all confounders, as long as a valid instrument exists—making them a cornerstone of 'natural experiments' in economics and epidemiology.
Why this is interesting
How can we prove that a new drug really works when we can't run a randomized trial? The answer may be hiding in an unlikely variable.