Why the tips of“Warning - singleton groups detected. 2533 observation(s) not used.” always appears when I use xtivreg2 to calculate the total factor productivity?
The process is as follows.
xtivreg2 lnY lnL (lnK=l.lnK),gmm2s fe robust
Warning - singleton groups detected. 2533 observation(s) not used.
no observations
r(2000);
Thank you in advance. I am happy to provide more information.
Zhangningning
