This is an embarrassing question, but I would appreciate an answer.

I have a panel of individuals surveyed in two time periods. There is quite a bit of missing data on both x/y variables. Xtset says panel is strongly balanced. I run FE but the output table suggests that for some groups there are less than 2 observations? and N of groups multiplied by 2 is not equal to Number of obs. I was assuming stata would kick out individuals for which there are no obs for one of the two years and would only take those that have obs for two time periods for whom there was a change? Could someone please explain this?

Here is an example of output:

Fixed-effects (within) regression Number of obs = 23,881
Group variable: caseid Number of groups = 13,364

R-sq: Obs per group:
within = 0.0377 min = 1
between = 0.0027 avg = 1.8
overall = 0.0000 max = 2