What is a hierarchical Bayesian model?
hard- Multi-level model: parameters vary across groups j and are themselves drawn from a shared 'population' distribution ~ .
- Partial pooling: group estimates borrow strength from each other via the shared prior — automatic shrinkage.
- Uses: hospital effects, per-user models, radon by county (Gelman's classic example).
- Solves the 'estimate 10,000 individual users' problem: no-pool overfits, full-pool ignores individuality, partial-pool balances them.