您好，我是第一次尝试DID-PSM模型
lev roa largestholderrate toptenholdersrate想采用滞后一期的数据 ，想请教一下我的代码问题出在哪里
diff il, t(treated) p(t) cov(reg l.lev growth age l.roa l.largestholderrate l.toptenholdersrate eqid) report
diff il, t(treated) p(t) cov(reg l.lev growth age l.roa l.largestholderrate l.toptenholdersrate eqid) kernel id(stock)

错误提示：
factor-variable and time-series operators not allowed
(error in option cov())