stata中，如何按月差分

想求变量的月度同比增长量：比如2007m1=msize(2007m1)-msize(2006m1)。由于有的年份有数据缺失，因此不能用如下命令：by cd:gen dmsize=msize-L12.msize
cd trdmnt mtrad msize mouth
2 2006-01 3.317e+09 1.32e+07 2006m1
2 2006-02 4.249e+09 1.46e+07 2006m2
2 2006-03 5.601e+09 1.89e+07 2006m3
2 2006-04 8.254e+09 1.80e+07 2006m4
2 2006-05 9.734e+09 1.72e+07 2006m5
2 2006-06 5.770e+09 1.65e+07 2006m6
2 2006-07 5.269e+09 1.62e+07 2006m7
2 2006-08 5.164e+09 2.02e+07 2006m8
2 2006-09 5.223e+09 2.21e+07 2006m9
2 2006-10 4.885e+09 2.39e+07 2006m10
2 2006-11 1.238e+10 3.50e+07 2006m11
2 2006-12 1.633e+10 5.02e+07 2006m12
2 2007-01 2.735e+10 4.98e+07 2007m1
2 2007-02 1.402e+10 4.80e+07 2007m2
2 2007-03 2.057e+10 5.40e+07 2007m3
2 2007-04 2.580e+10 5.88e+07 2007m4
2 2007-05 4.764e+10 9.47e+07 2007m5
2 2007-06 4.962e+10 9.33e+07 2007m6
2 2007-07 4.104e+10 1.38e+08 2007m7
2 2007-08 6.974e+10 1.65e+08 2007m8
2 2007-09 5.149e+10 1.57e+08 2007m9

已经解决：by cd:gen dmsize=msize-L12.msize就可以