/* Percent Difference Smoothing */
/* (Forecast Smoothing) */
new; cls;
let data[12,1]=
23
25
36
31
26
28
48
36
31
42
53
43
;
t=seqa(1,1,rows(data));

library pgraph;
graphset;
_pltype=6;
_plegctl=1;
_plegstr="actual\000predicted";
title("Percent Difference Smoothing");
nperiod=4;
xy(t,data~pdiff(data,nperiod));




proc pdiff(x,nperiod);
   local n,m,i;
   n=rows(x);
   m=zeros(n,1);
   i=nperiod+2;
   do while i<=n;
      m[i]=x[i-1]*(x[i-1]/x[i-1-nperiod]); 
      i=i+1;
   endo;
   m[1:nperiod+1]=miss(zeros(nperiod+1,1),0);
   retp(m);
endp;