data one;
   input IQ study grade;
   iqstudy=iq*study;
   cards;
 105 10 75
 110 12 79
 120  6 68
 116 13 85
 122 16 91
 130  8 79
 114 20 98
 102 15 76
;
run;
data grid;
   do iq=100 to 130 by 5;
   do study=4 to 20 by 4;
   iqstudy=iq*study;
      grade=.;
      output;
   end;
   end;
run;
data both;
   set one grid;
run;
proc reg data=both;
   model grade=iq study ;
   output out=add p=p;
   model grade=iq study iqstudy;
   output out=mult p=p;
run;
data add; set add;  if grade=.; run;
symbol1 value=dot i=join;
proc gplot data=add;
   title "additive";
   plot p*iq = study;
   plot p*study = iq;
run;
proc print data=two;run;

data mult; set mult;  if grade=.; run;
symbol1 value=dot i=join;
proc gplot data=mult;
   title "multiplicative";
   plot p*iq = study;
   plot p*study = iq;
run;
proc print data=two;run;