Machine Learning Algorithms - Second Edition

Unsupervised learning

This approach is based on the absence of any supervisor and therefore of absolute error measures. It's useful when it's necessary to learn how a set of elements can be grouped (clustered) according to their similarity (or distance measure). For example, looking at the previous graph, a human being can immediately identify two sets without considering the colors or the shapes. In fact, the circular dots (as well as the triangular ones) determine a coherent set; it is separate from the other one much more than how its points are internally separated. Using a metaphor, an ideal scenario is a sea with a few islands that can be separated from each other, considering only their mutual position and internal cohesion. Clearly, ...