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 insulation data, no evidence of skewness or outliers exists.