Data Analysis with Microsoft Excel: Updated for Office 2007

Chapter 8 Regression and Correlation 331

The plot shows that most of the positive residuals tend be located at the

lower and higher temperatures and most of the negative residuals are con-

centrated in the middle temperatures. This may indicate a curve in the data.

The large fi rst observation is infl uential here. Without it, there would be less

indication of a curve.

Testing for Normal Distribution of the Residuals

The next diagnostic is a normal plot of the residuals. The Analysis ToolPak

does not provide this chart, so we’ll create one with StatPlus.

To create a Normal Probability Plot of the residuals:

1 Return to the Regression Statistics worksheet.

2 Click Single Variable Charts from the StatPlus menu and then click

Normal P-Plots.

3 Click the Data Values button.

4 In the Input Options dialog box, click the Use Range References

option button and then select the range C24:C40. Verify that the Range

Include a Row of Column Labels checkbox is selected and click OK.

5 Click the Output button, verify that the As a New Chart sheet option

button is selected, and type Residual Normal Plot in the accompa-

nying text box. Click the OK button.

6 Click the OK button to start generating the Normal Probability plot.

See Figure 8-17.

Figure 8-17

Normal

probability

plot of

residuals