data a1;
   infile 'U:\.www\datasets512\Ch11ta06.dat';
   input cost lotsize;
proc print data=a1;
run;

*First plot the data;
symbol1 v=circle i=sm70 c=black;
proc sort data=a1; by lotsize;
proc gplot data=a1;
   plot cost*lotsize/frame;
run;

*Define new variable to allow slope to change;
Data a1; set a1; 
   if lotsize le 500 
     then cslope=0;
   if lotsize gt 500 
     then cslope=lotsize-500;

*Alternative cslope=(lotsize gt 500)*(lotsize-500);

proc print data=a1;
run;

*Fit model and save predicted values;
proc reg data=a1;
   model cost=lotsize cslope;
   output out=a2 p=costhat;
run;

*Plot piecewise linear model;
symbol1 v=circle i=none c=black;
symbol2 v=none i=join c=black;
proc sort data=a2; by lotsize;
proc gplot data=a2;
   plot (cost costhat)*lotsize/frame overlay;
run;