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Greene is right - period fixed effects are the same thing as period dummies, and you cannot have both.

Thanks Gareth. I did not have both. It is just that because the variables are time invariant, I was not comfortable with the results from a specification that included time fixed effects.

If I'm interpreting the original question correctly it's about whether you can have time-invariant variables and period fixed effects, and the answer is yes. What you can't have are time-invariant variables and cross-section fixed effects, or cross-section invariant variables and period effects. Gl...

We know that since time-invariant variables are collinear with fixed effects their coefficients cannot be estimated in FEM. However, when I include my time invariant variables (dummies representing state specific balance budget requirements, BBRs) with period fixed effects only I get estimates (see ...