ata it has never seen.*

To avoid overfitting, we must:

1. Split our data into a **training set** and a **test set**.
2. Use the **training set** only to train our model.
3. Use the **test set** only to measure model performance.


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Fitting a polynomial of degree **1**
&lt;img src="13-Learning_files/figure-html/unnamed-chunk-3-1.svg" style="display: block; margin: auto;" /&gt;

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Fitting a polynomial of degree **2**
&lt;img src="13-Learning_files/figure-html/