Number of observations in different panel data regressions Given a balanced two-period panel data, with lets say 1000 observations on 500 individuals.

When you estimate a pooled OLS regression and first-differences regression is there a standard in the economics literature on how you should report number of observations. 

It seems most natural for me to report 500 observations in the case of first-differences.

This might seem trivial. But better follow any standard if it exists.
It is conventional to report both the total number of observations (1000 in your case) and the number of groups (500 in your case). If you want or are allowed to report only one number as the number of observations, it is suitable to report 1000 for POLS because POLS doesn't mind the fact that it is a panel data set. For FD, it is slightly confusing to me to say that the number of observations is 500. I would probably ask again of the writer what 500 means.