A histogram of the residuals shows the distribution of the residuals for all observations. Use the histogram as an exploratory tool to learn about the following characteristics of the data:

·    Typical values, spread or variation, and shape

·    Unusual values in the data

The histogram of the residuals should be bell-shaped. Use this plot to look for the following:

Because the appearance of the histogram can change depending on the number of intervals used to group the data, use the normal probability plot and goodness-of-fit tests to assess whether the residuals are normal.

For the health care center data, the residuals have two groups. This pattern often means not enough data is in the set for normal approximation theory to apply. Confidence intervals for predictions are probably inaccurate.

The bin from 1.875 to 2.125 contains observation 58. Observation 58 is the point with the large standardized residual that is in the table that shows the fits and diagnostics for unusual observations.