er `\(5\)` as the .hi-slate[validation set] produces MSE.sub[k=5].

---

Relative to the validation set approach:
- Uses all the data, so not as sensitive to exactly which observations end up in the validation set.
- So produces a lower-variance estimate of the test-set MSE.


---
layout: false
# Summary

.smaller[
.pull-left[
[**Prediction vs. causal inference**](#goal)
- In causal inference we want to estimate the treatment effect `\(\hat{\beta}\)`.
- In prediction problems we want to estimate the outcome