Dear All,
I have a couple of questions. I have found something similar in older posts but I have not clear the answer. Thank you in advance for any help
I want to estimate the following model by fixed effects
Yjt = b*Xjt + Ujt
Yjt = birth rate in county j in year t
Xjt county level covariates
there are 615 counties and 8 years
Question 1
I estimated the model by using
1) xtreg Yjt Xjt, fe i(county)cluster(county)
2) xi: reg Yjt Xjt i.county, cluster(county)
3) areg Yjt Xjt, absorb(county) cluster(county)
The coefficients estimates are the same in the 3 specifications.
The standard errors are exactly the same in 2) and 3) (as it should be)
but slightly different fron 1). I understand that there is some difference in the degrees of freedom adjustment, but I am not sure...
Question 2
I re-estimated the model by weighting each observation by the county population in each year, POPjt
1) xtreg Yjt Xjt [aweight=POPjt], fe i(county)cluster(county)
2) xi: reg Yjt Xjt i.county [aweight=POPjt], cluster(county)
3) areg Yjt Xjt [aweight=POPjt], absorb(county) cluster(county)
agai, 2) and 3) give exactly the same result, but 1) cannot be estimated because STATA says that weights must be constant whithin panel.
I do not understand this and why xtreg cannot estimate the weighted regression while the dummy variable regression allows weights
thank you for any help
Sara
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