请各位大佬看一下为什么用tvdiff做多期DID的平行趋势检验显示no observation了呢？大概有154万个观测值，控制变量有10个，工企98-05的非平衡面板，下面是具体命令. tvdiff tfp_op D $Xs,model(fe) pre(4) post(7) test_tt graph vce(robust)
(329,803 real changes made)
(1,140 observations deleted)
       panel variable:  newid (unbalanced)
        time variable:  year, 1998 to 2005, but with gaps
                delta:  1 unit
(474,215 missing values generated)
(812,709 missing values generated)
(1,033,098 missing values generated)
(1,197,649 missing values generated)
(1,326,287 missing values generated)
(1,418,118 missing values generated)
(1,486,262 missing values generated)
(1,197,649 missing values generated)
(1,033,098 missing values generated)
(812,709 missing values generated)
(474,215 missing values generated)
no observations