* 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  16.92
   90  16.08   105 17.14   144  18.85
  178  18.63   210 19.30   233  25.77
  256  28.44   300 35.46   400  45.26

 proc nlin method=marquardt;
 parameters  g = 18.63
             b = 38.88
             t = 5.1818;
 if x <= t then do;          *left plateau;
             model y = g ;
             der.g = 1;
             der.b = 0;
             der.t = 0;
           end;
 else do;                    *post-threshold;
         model y = g + b*(x-t);
         der.g = 1;
         der.b = x-t;
         der.t = -b;
       end;
 run;