在做面板数据的DID，因变量是ln_CE，policy表示政策处理，试点城市policy为1，非试点城市policy为0，city_id为城市代码，year为年份使用命令didregress (ln_CE) (policy), group(city_id) time(year)后报错：invalid group specification
    None of the groups defined by city_id are treated.


参考了didregress的示范数据hospdd中procure的格式，换成了policy_new变量，
gen policy_new = 1 if policy ==1 & year >= 2018
replace policy_new = 0 if policy_new ==.

仍然报错：
invalid group specification
    None of the groups defined by city_id are treated.


求大佬指点，为什么一直显示没有城市被处理啊，有变量可以将控制组和处理组做区分啊