FIGURE A14.4 Minitab 3D Scatterplots Dialog Box

A14.2 MINITABGenerating a Multiple Regression EquationIn Appendix A13.2, instructions are provided for usingMinitab for simple linear regression. The same set ofinstructions is valid in using Minitab for multiple regres-sion. To carry out a regression analysis for the OmniPowersales data, open the OMNI.MTW worksheet and selectStat Regression Regression.

1. Enter C1 or Sales in the Response: edit box and C2 orPrice and C3 or Promotion in the Predictors: edit box.Click the Graphs button.

2. In the Residuals for Plots: edit box, select the Regularoption button. To check for normality, select theHistogram of residuals check box. In the Residualsversus the variables: edit box, select C2 or Price andC3 or Promotion. Click the OK button to return to theRegression dialog box.

3. Click the Results option button. In the Regression-Results dialog box, click the In addition, the full tableof fits and residuals option button. Click the OK but-ton to return to the Regression dialog box.

4. Click the Options button. (If the data have been col-lected over time, under Display, select the Durbin-Watson statistic check box.) In the Prediction intervalsfor new observations: edit box, enter 79 400. Enter 95in the Confidence level: edit box. Click the OK buttonto return to the Regression dialog box. Click the OKbutton.

Using Minitab for a Three-DimensionalPlotYou can use Minitab to construct a three-dimensional plotwhen there are two independent variables in the regressionmodel. To illustrate the three-dimensional plot with theOmniPower sales data, open the OMNI.MTW worksheet.Select Graph 3D Scatterplot.

1. In the 3D Scatterplots dialog box (see Figure A14.4),select the Simple button. Click the OK button.

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FIGURE A14.5 Minitab 3D Scatterplot - SimpleDialog Box

2. In the 3D Scatterplot - Simple dialog box (see FigureA14.5), enter C1 or Sales in the Z variable: edit box,C2 or Price in the Y variable: edit box, and C3 orPromotion in the X variable: edit box. Click the OKbutton.

Using Minitab for Logistic RegressionTo illustrate the use of Minitab for logistic regression withthe credit card upgrade example on page 605, open theLOGPURCH.MTW worksheet. To perform a logisticregression, select Stat � Regression � Binary LogisticRegression.

In the Binary Logistic Regression dialog box (seeFigure A14.6), in the Response: edit box, enter C2 orPurchase. In the Model: edit box, enter C1 or Spendingand C3 or Extra. In the Factors: edit box, enter C3 or Extrabecause it is a categorical variable. Click the OK button.

Using Minitab for Dummy Variables and InteractionsIn order to perform regression analysis with dummy vari-ables, the categories of the dummy variable are coded as 0and 1. If the dummy variable has not already been recodedas a 0–1 variable, Minitab can recode the variable. As anillustration with the data of Table 14.5 on page 592, openthe HOUSE3.MTW worksheet. In this worksheet, the fire-place variable in column C3 has been entered as Yes andNo. To recode this variable using Minitab, select Calc �Make Indicator Variables. In the Indicator variables for:edit box, enter Firepl or C3. In the Store results in: editbox, enter C4 C5, because you need to specify a columnfor each possible definition of the dummy variable eventhough only one column (C5) will be used in the regressionanalysis. Click the OK button. Observe that No is coded as1 in C4, and Yes is coded as 1 in C5. Enter the labelFireplace for C5.

To define an interaction term that is the product ofheating area and the dummy variable fireplace, select Calc� Calculator. In the Store result in variable: edit box enterC6. In the Expression: edit box, enter Size * Fireplaceor C2 * C5. Click the OK button. C6 now contains a newX variable that is the product of C2 and C5. Enter a labelfor C6.

FIGURE A14.6 Minitab Binary Logistic RegressionDialog Box

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