%newsas(tiretest);
TITLE 'TIRE TESTING (NWK 10.15)';
DATA TIRES;
   INPUT X Y MAKE $;
   Label X = 'Cruising speed'
         X2= 'Make (dummy)'
         X3= 'X * X2'
         Y = 'Cost per mile';
   If MAKE = 'A' then X2 = 1;
                 else X2 = 0;
   X3 = X * X2;
datalines;
10  9.8 A
20 12.5 A
20 14.2 A
30 14.9 A
40 19.0 A
40 16.5 A
50 20.9 A
60 22.4 A
60 24.1 A
70 25.8 A
10 15.0 B
20 14.5 B
20 16.1 B
30 16.5 B
40 16.4 B
40 19.1 B
50 20.9 B
60 22.3 B
60 19.8 B
70 21.4 B
Proc Plot data=TIRES;
   Plot Y * X = MAKE;
Proc Reg data=TIRES;
Parallel:  Model Y = X X2    / ss1;
           Output out=R2  R=resid P=predict;
Interact:  Model Y = X X2 X3 / ss1;
           Output out=R3  R=resid P=predict;
Proc Plot data=R2;
    Plot resid * X = MAKE;
    Title2 'Residuals from equal slopes model';
Proc Plot data=R3;
    Plot resid * X = MAKE;
    Title2 'Residuals from model with interaction';
    run;