* SAS code to fit mercury data from Table 7.1;
 options linesize=70;

 data mercury;
 input hgconc y @@;
    x = log(hgconc);
 cards;
  2.5  13.57    22 15.33    60  4.09  
   90   4.50   105  4.65   144  4.97 
  178  5.18    210  5.35   233  5.45 
  256  5.55    300 35.46   400 45.26

 proc nlin method=marquardt ;
 parameters  g0 = 12.73
             g1 =  1.06
             b1 = 38.88
             t = 5.1818 ;
 if x <= t then do;     * left linear segment;
             model y = g0 + g1*x ;
             der.g0 = 1;
             der.g1 = x;
             der.b1 = 0;
             der.t  = 0;
           end;
 else do;               * right linear segment;
         model y = g0 + g1*t + b1*(x-t);
         der.g0 = 1;
         der.g1 = t;
         der.b1 = x-t;
         der.t  = g1 - b1
       end;
 run;