我已经将mem 设置到1G，变量20个，但还是得不到结果。这怎么处理呢？跪求解答。

Building GMM instruments.no room to add more variables because of width
    An attempt was made to add a variable that would have increased the memory required to store an observation beyond what is currently
    possible.  You have the following alternatives:

     1.  Store existing variables more efficiently; see help compress.

     2.  Drop some variables or observations; see help drop.  (Think of Stata's data area as the area of a rectangle; Stata can trade off
         width and length.)

     3.  Increase the amount of memory allocated to the data area using the set memory command; see help memory.