**this block of code reads in two data files that have the same ID variables
but different other variables:
data set ONE has playerID yearID Y. File TWO has playerID yearID and X.
The data need to be sorted by the two ID variables before merging them into a
third data set. This third
data set will have 4 columns" playerID yearID y x;
data
one;
data
two;proc
sort data=two;data three;