"Warning - singleton groups detected" error

[Lukas Novak]

Dear all,

I am posting here with hope that someone could help me with my error. I am running an IV FE regression with panel data using the command xtivreg2. However, I keep getting an error saying "xtivreg2 pgemplst [education age i.sex i.corigin] (mcs =i.trauma##arrival_year) [aw=phrf], fe
Warning - singleton groups detected. 1358 observation(s) not used." I would appreciate if someone could help me with dealing with singleton groups! many thanks

[Miftah Zikri]

Hi Lukas, You may attempt to use the tsset or "xtset id year" before run xtivreg2. After xtset you can test xtivreg2 command again. If the same error occurred, then use xtset year. The singleton groups detected due to prior incorrectly setting for panel data xtset or tsset.