Losing groups while running a regression

[Jari Landman]

Hello,

I am trying to make a panel data regression. My group variable (cntry) has 41 countries. But when I run a regression the number of groups is reduced to 18. I cannot find anything about this and stata does not say anything about it or why these groups are left out. Whaat are possible reasons that these groups are not taken into account when running the regression?

[Carlo Lazzaro]

Jari:
welcome to this forum.
In all likelihhod, your omitted groups have a missing values problem.
Please note that Stata omits by default observations with missing values in any variable.