## 12.1 Outliers and Outlier Analysis

Let us first define what outliers are, categorize the different types of outliers, and then discuss the challenges in outlier detection at a general level.

### 12.1.1 What Are Outliers?

Assume that a given statistical process is used to generate a set of data objects. An **outlier** is a data object that deviates significantly from the rest of the objects, as if it were generated by a different mechanism. For ease of presentation within this chapter, we may refer to data objects that are not outliers as “normal” or expected data. Similarly, we may refer to outliers as “abnormal” data.

Example 12.1

Outliers

In Figure 12.1, most objects follow a roughly Gaussian distribution. However, the objects in region *R* are significantly ...