Skill: Identify outliers informally

An outlier is a data value that is noticeably far from the rest of the data. To identify one:

1. Look at the values in order.
2. Find the main cluster, or group, of values close together.
3. Look for a value separated from that cluster by a large gap.
4. Decide whether that value is unusual in the context.

Example: The numbers of books read by 8 students in one month are

$$2,\ 3,\ 3,\ 4,\ 4,\ 5,\ 5,\ 15$$

Step 1: Find the main cluster.
Most of the values are between $2$ and $5$. These numbers are close together, so they form the main cluster.

Step 2: Look for a value far from the cluster.
The value $15$ is much greater than the other values. There is a large gap between $5$ and $15$.

Step 3: Identify the outlier.
The outlier is

$$\boxed{15}$$

It means one student read many more books than the rest. The outlier is not automatically the greatest or least value; it is the value that is noticeably separated from the main pattern.