What are outliers in scatter plots?

Scatter plots often have a pattern. We call a data point an outlier if it doesn't fit the pattern.

Consider the scatter plot above, which shows data for students on a backpacking trip. (Each point represents a student.)

Notice how two of the points don't fit the pattern very well. These points have been labeled Brad and Sharon, which are the names of the students they represent.

Sharon could be considered an outlier because she is carrying a much heavier backpack than the pattern predicts.

Brad could be considered an outlier because he is carrying a much lighter backpack than the pattern predicts.

Key idea: There is no special rule that tells us whether or not a point is an outlier in a scatter plot. When doing more advanced statistics, it may become helpful to invent a precise definition of "outlier", but we don't need that yet.