各位高人，我在用stata做事件研究时，运行至forvalues i=1(1)3770 {l id comid if id==`i' & dif==0
reg ret mret if id==`i' & estimation_window==1
predict p if id==`i'
replace predicted_return = p if id==`i' & event_window==1
drop p
}
到一半时出现错误insufficient observations（r2001）：

        Keywords:  r(2001)
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[P]     error . . . . . . . . . . . . . . . . . . . . . . . . Return code 2001
        insufficient observations;
        You have requested some statistical calculation, and while there
        are some observations, the number is not sufficient to carry out
        your request.

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请各位达人帮忙解决问题啊，万分感谢！