Hi all,

I am running a regression using "reghdfe", with state fixed effect (FE), time FE, and state linear/quadratic/cubic time trend. I wonder if my codes are correct:

gen yearsq=year*year
gen yearcb=year*year*year

reghdfe Y X1 X2 X3, absorb (state year i.state#c.year) for linear time trend

reghdfe Y X1 X2 X3, absorb (state year i.state#c.year i.state#c.yearsq) for quadratic time trend

reghdfe Y X1 X2 X3, absorb (state year i.state#c.year i.state#c.yearsq i.state#c.yearcb) for cubic time trend

Are there differences if I code like this:

reghdfe Y X1 X2 X3 i.state#c.year i.state#c.year#c.year i.state#c.year#c.year#c.year, absorb (state year)


Thank you very much if you could help me to explain this.

Thien.