以下是我的程序

proc reg data=test outest=regtest;
model rr=mktret D1 D2 D3 D4 D5 D6;/*event day*/
by sktno_v;
run;

请问为何在跑回归时，没有by sktno_v时，可以run，但加上sktno_v（按照每间公司）

Log就会出现ERROR: No valid observations are found.

Log 如下

NOTE: 496 observations read.

NOTE: 496 observations have missing values.

NOTE: 0 observations used in computations.

ERROR: No valid observations are found.

NOTE: The above message was for the following by-group:

      sktno_v=1102