A team of mental health researchers wishes to compare three
treatments (A, B, and C) for depression.  They would also like to study the
relationship between age and treatment effectiveness, as well as the
interaction (if any) between age and treatment.  Each member of a
 simple random sample of 36 patients with depression was randomly assigned
to one of the 3 treatments.  The variables in the dataset are:

   eff --  the treatment effectiveness score (the higher the better)
   age --  age in years at nearest birthday
   trt --  treatment (A, B, or C)