An admittedly small but nonetheless important point emerged in a recent StataList discussion (here). While the original query wondered whether a decision between the "reg" and "xtreg" commands pivoted on whether panel data were balanced or unbalanced, a very helpful commenter quickly made clear the question's pivotal (and mistaken) assumption. To be sure, while the "reg" and "xtreg" commands may yield the same or similar results in certain conditions, they estimate different models and different parameters. In general, where panel data are involved (balanced or unbalanced), as the commenter notes,"-reg- is simply inappropriate."
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