The data set EXP.SUR contains the variable WT6MOS, but the EXP.RESULTS data set does not. 

                      The EXP.SUR Data Set                     2

       ID     treat     initwt    wt3mos    wt6mos    age

       14    surgery    203.60    169.78    143.88     38
       17    surgery    171.52    150.33    123.18     42
       18    surgery    207.46    155.22       .       41