In this chapter, you will learn how to apply unsupervised learning techniques to identify patterns and structure within datasets.

Unsupervised learning techniques are a valuable set of tools for exploratory analysis. They bring out patterns and structure within datasets, which yield information that may be informative in itself or serve as a guide to further analysis. It's critical to have a solid set of unsupervised learning tools that you can apply to help break up unfamiliar or complex datasets into actionable information.

We'll begin by reviewing Principal Component Analysis (PCA), a fundamental data manipulation technique with a range of dimensionality reduction applications. Next, we will discuss k-means clustering, a widely-used and approachable unsupervised learning technique. Then, we will discuss Kohenen's Self-Organizing Map (SOM), a method of topological clustering that enables the projection of complex datasets into two dimensions.

Throughout the chapter, we will spend some time discussing how to effectively apply these techniques to make high-dimensional datasets readily accessible. We will use the UCI Handwritten Digits dataset to demonstrate technical applications of each algorithm. In the course of discussing and applying each technique, we will review practical applications and methodological questions, particularly regarding how to calibrate and validate each technique as well as which performance measures are valid. To recap, then, we will be covering the following topics in order: