filename ped561 'p561.dat';
options center linesize=78 pagesize=65 nodate;
data;
infile ped561;
input id incen st y;
interact = incen * st;
if incen = 0 then contr = -1; else contr = 1;
inter = contr * st;
proc reg;
model y = incen st interact;
model y = incen st;
model y = contr st inter;
model y = contr st;
model y = incen;
model y = st;
run;
quit;