# Introduction to Regularization

Understand the difference between underfitting and overfitting, as well as the necessity and use of regularization.

Let's start with understanding the difference between underfitting and overfitting.

## Underfitting vs. overfitting

Let's say we have the following plots (shown below):

Left plot: Underfitted model with low accuracy score (R-squared) and higher error (SSE: sum of squared error).

Middle plot: Model with average accuracy score and error. Can this be improved?

Right plot: The overfitted model has a very high accuracy score and low error (practically R-squared = 1 and SSE = 0 since the fitted line passes through all the data points).

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