使用下面语句计算所有样本CAAR时，t值和p值缺失是为什么？
forvalues j=1/61{
qui sum stkcd
local Ns=r(N)
egen a_mean_`j'=mean(CAR_`j')
egen a_sd_`j'=sd(CAR_`j')
gen a_t_`j'=a_mean_`j'/(a_sd_`j'/sqrt(`Ns'))
gen a_p_`j'=ttail(`Ns', abs(a_t_`j'))*2
drop a_sd_`j'
}
keep a_*