214 PART 5 Looking for Relationships with Correlation and Regression

Straight-line regression is appropriate when all of these things are true:»

» ^{You’re interested in the relationship between two — and only two —}

numerical variables. At least one of them must be a continuous variable

that serves as the dependent variable (Y).»

» ^{You’ve made a scatter plot of the two variables and the data points seem to}

lie, more or less, along a straight line (as shown in Figures 16-1a and 16-1b).

You shouldn’t try to fit a straight line to data that appears to lie along a curved

line (as shown in Figures 16-1c and 16-1d).»

» ^{The data points appear to scatter randomly around the straight line over the}

entire range of the chart, with no extreme outliers (as shown in Figures 16-1a

and 16-1b).

FIGURE 16-1:

Straight-line

regression is

appropriate for

both strong and

weak linear

relationships

(a and b), but not

for nonlinear

(curved-line)

relationships

(c and d).

© John Wiley & Sons, Inc.