st: xtivreg2 singleton groups identification or definitions

["Shao,Diana" <diana.shao@warrington.ufl.edu>]

Hello all, 
I'm using the -xtivreg2- to run a regression that considers firm fixed effect and two-way cluster (firm year). So -xtivreg- seems to be a good choice. I have read through several posts on statalist that the singletons groups would be detected and not used in running xtivreg2.  One of the posts says "a singleton is a group in which there is only one observation". I use "xtset gvkey" on my data so I assume that a singleton group for my sample would be a firm that only enters the sample once. When running xtivreg2, stata says that 1592 singleton groups detected. But when I manually sort firm and then by firm: gen number=_N and count if number==1, stata reports 1406. I wonder if anyone knows how to explain this discrepancy between the counting of singleton groups. Did I understand the definition of singleton groups wrong? Or did I count in a wrong way? 
Or if anyone knows how to control for firm fixed effect and at the same time clustering both year and firm, that'd be greatly appreciated too. 
Many thanks to all of you. 
Anxiously waiting for replies, 
Diana Shao

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