data one;
  input fert yield @@;
  cards;
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;

proc plot;
  plot yield*fert;
run;

proc reg;
  model yield=fert;
run;

data one; set one;
  lack_of_fit=fert;
run;

proc glm;
  class lack_of_fit;
  model yield=fert lack_of_fit;
run;

data two;
  input fert yield @@;
  cards;
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;

proc plot;
  plot yield*fert;
run;

proc reg;
  model yield=fert;
run;

data two; set two;
  lack_of_fit=fert;
run;

proc glm;
  class lack_of_fit;
  model yield=fert lack_of_fit;
run;