options ls=72;
data a;
input x y @@;
cards;
 8 .49  8 .49 10 .48 10 .47 10 .48 10 .47 12 .46 12 .46 12 .45 12 .43
14 .45 14 .43 14 .43 16 .44 16 .43 16 .43 18 .46 18 .45 20 .42 20 .42
20 .43 22 .41 22 .41 22 .40 24 .42 24 .40 24 .40 26 .41 26 .40 26 .41
28 .41 28 .40 30 .40 30 .40 30 .38 32 .41 32 .40 34 .40 36 .41 36 .38
38 .40 38 .40 40 .39 42 .39
proc plot; plot y*x;
proc nlin method=marquardt data=a;
parms alpha=0 to 1 by .2
      beta=0 to 1 by .2;
exp1=exp(-beta*(x-8.));
model y=alpha+(.49-alpha)*exp1;
der.alpha=1.-exp1;
der.beta=-(.49-alpha)*(x-8.)*exp1;
output out=outa p=yhat r=yresid;
proc plot; plot yresid*x yhat*x;
run;