244 PART 5 Looking for Relationships with Correlation and Regression

Figure 17-3 shows two graphs you can optionally request that help you determine

or diagnose whether these assumptions are met, so they are called diagnostic graphs

(or plots).»

» ^{Figure 17-3a provides an indication of variability of the residuals. To interpret}

this plot, visually evaluate whether the points seem to scatter evenly above

and below the line, and whether the amount of scatter seems to be the same

across the left, middle, and right parts of the graph. That seems to be the case

in this figure.»

» ^{Figure 17-3b provides an indication of the normality of the residuals. To}

interpret this plot, visually evaluate whether the points appear to lie along the

dotted line or are noticeably following a curve. In this figure, the points are

consistent with a straight line except in the very lower-left part of the graph.

Determining how well the

model fits the data

Several calculations in standard regression output indicate how closely the model

fits your data:»

» ^{The residual SE is the average scatter of the observed points from the fitted}

model. You want them to be close to the line. As shown in Figure 17-2, the

residual SE is about 11 mmHg.

FIGURE 17-3:

Diagnostic

graphs from a

regression.

© John Wiley & Sons, Inc.