t MSE.


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# Cross-validation
## *k*-fold cross-validation

With `\(k\)`-fold cross validation, we estimate test MSE as
$$
`\begin{align}
  \text{CV}_{(k)} = \dfrac{1}{k} \sum_{i=1}^{k} \text{MSE}_{i}
\end{align}`
$$
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&lt;img src="13-Learning_files/figure-html/plot-cvk-0a-1.svg" style="display: block; margin: auto;" /&gt;

Our `\(k=\)` 5 folds.
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&lt;img src="13-Learning_files/figure-html/plot-cvk-0b-1.svg" style="display: block; margin: auto;" /&gt;

Each fold tak