DecisionTrees

At a basic level, machine learning is about predicting the future based on the past. For instance, you might wish to predict how much a user Alice will like a movie that she hasnt seen, based on her ratings of movies that she has seen. This means making informed guesses about some unobserved property of some object, based on observed properties of that object.

The first question well ask is: what does it mean to learn? In order to develop learning machines, we must know what learning actually means, and how to determine success (or failure). Youll see this question answered in a very limited learning setting, which will be progressively loosened and adapted throughout the rest of this book. For concreteness, our focus will be on a very simple model of learning called a decision tree.