In order to enter interaction terms into a regression model in Minitab, you have to first create column(s) in the worksheet that contain the interaction term(s).
- Select Calc >> Calculator...
- In the box labeled Store the result in variable, specify the column (or the name of the new variable, x1x2, for example) in which you want to store the interaction term.
- In the box labeled Expression, multiply the two predictor variables that go into the interaction terms. For example, if you want to create an interaction between x1 and x2, use the calculator to multiply them together: 'x1'*'x2'.
- Select OK. The new variable, x1x2, should appear in your worksheet.
The data set birthsmokers.txt contains data on the birthweight (y = Wgt), gestation length (x1 = Gest) and (x2 = Smoke, 1 if mother smoked, 0 if not) of babies born to 32 mothers. If you wanted to fit a multiple regression model that allowed for an interaction between gestation length and smoking, you'd first have to create a variable in your worksheet, GestSmoke say, that contained the interaction term. Use Minitab's calculator to create the interaction term in your worksheet.
Minitab Dialog Box
Sample of Resulting Minitab Worksheet