Dear,

I am estimating a panel data fixed effects model (both year and industry). I am aware I can either run -reg or -xtreg for this regression.

Code:
reg board sales i.year i.industry, robust
Code:
xtset year
xtset industry
xtreg board sales, fe vce(robust)
Both the coefficient and the SE seem to differ for the two different models. What am I doing wrong and what is the right approach?

In addition to that, when would I be right to leave to constant out. That is, run:

Code:
reg board sales i.industry i.year, noconstant robust
When doing so, the coefficient for sales differs as well.

Thanks in advance.