I only have 23 obervations. I heard that GMM-IV is usually for large sample. Hence should I use GMM-IV or 2TSLS?
Besides the sample size, what should I also consider when deciding between GMM-IV and 2TSLS?
Thank you
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Both GMM and 2SLS (do not write 2TSLS - TSLS or 2SLS is short-hand for two-stage least squares) are only justified asymptotically, so in large samples, so that will not make a difference.
The key distinction is that GMM uses a more general weighting matrix that yields the efficient estimator also under conditional heteroskedasticity, whereas 2SLS uses a weighting matrix that leads to an efficient estimator only under homoskedasticity.
Both are consistent in either case, however (whether or not consistency is a property that is of much relevance to you, given such a small sample, is another matter).
answered Jul 13 '15 at 6:09
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$\begingroup$ If I understood you correctly, you are saying that GMM-IV can be used as a reliable estimator for small samples? I have 25 quarterly observations and my intention is to use GMM-IV. SInce I'm not an econometrician by profession, I want to be sure that GMM-IV will still provide reliable estimate. $\endgroup$– QuirikSep 14 '15 at 8:34
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$\begingroup$ well...little data is a problem, there is no magic way out if you have so little information $\endgroup$ Sep 14 '15 at 8:37