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Re: st: RE: ivreg2/xtivreg2, clustering, and the covariance matrix of moment conditions


From   Augusto Cadenas <aug.cadenas@googlemail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: ivreg2/xtivreg2, clustering, and the covariance matrix of moment conditions
Date   Fri, 20 Feb 2009 23:27:26 -0500

Dear Mark,

thank you very much, this has clarified it. One more question:

>> For example, I have N=8, T=12, a balanced panel (N*T=96), and want to
>> have one cluster for each state (N), so M=8. The model I estimate has
>> time and state fixed effects and only one regressor beyond that, so
>> 1+7+11 coefficients to estimate. Is that because 19>8?
>
> If you are using xtivreg2, then you probably estimating a fixed effects
> model with 8 fixed effects and 11 time dummies.  The fixed effects are
> NOT explicitly estimated - as usual with the FE model, they are
> partialled out using the within transformation (demeaning).  Thus you
> are probably obtaining 1+11=12 explicit coefficient estimates.
>
> But your conclusion is basically correct.  Because 12 (number of
> coefficients, var-cov matrix of moment conditions is 12x12) > 8 (number
> of clusters, rank of the var-cov matrix of moment conditions), the
> var-cov matrix of moment conditions is not of full rank.

How does the number of instruments factor into this calculation? In
this specific example, I tried to partial out the year dummies, so I
should be (and actually am) fine. The remeining number of coefficients
is 1, which is greater than 8. But if I move to an overidentified
case, and increase the number of instruments, at some point I get the
message again. So, to be precise, I get the message with 9
instruments, but not with 8. So is the rule:

# of regressors + # of (excl) instruments - 2 < # of clusters

Thanks,

AC
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