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xtdpdsys or xtdpd is more concise way to write code for system GMM, but basically similar to xtabond2.
xtdpdsys or xtdpd can set the predetermined vars in "pre()" and endpgenous vars in "endog()", but they do not report sargan test and AR(2), need to use "estat sargan" and "estat abond" to get the postestimation, but xtabond2 automatically report these.
Here are from stata website for their difference:
http://www.stata-press.com/manuals/stata10/xtintro.pdf
b. New estimation command xtdpdsys fits dynamic panel-data models by using the Arellano–Bover/Blundell–Bond system estimator. xtdpdsys is an extension of xtabond and produces
estimates with smaller bias when the AR process is too persistent. xtpdsys is also more efficient than xtabond. Whereas xtabond uses moment conditions based on the differenced
errors in producing results, xtpdsys uses moment conditions based on differences and levels.
See [XT] xtdpdsys.
c. New estimation command xtdpd fits dynamic panel-data models extending the Arellano–Bond or the Arellano–Bover/Blundell–Bond system estimator and allows a richer syntax for specifying models and so will fit a broader class of models then either xtabond or xtdpdsys. xtdpd can be used to fit models with serially correlated idiosyncratic errors,
whereas xtdpdsys and xtabond assume no serial correlation. xtdpd can be used with models where the structure of the predetermined variables is more complicated than that assumed by xtdpdsys or xtabond. See [XT] xtdpd.d.
New postestimation command estat abond tests for serial correlation in the first-differenced
errors. See [XT] xtabond postestimation, [XT] xtdpdsys postestimation, and [XT] xtdpd postestimation.
e. New postestimation command estat sargan performs the Sargan test of overidentifying restrictions. See [XT] xtabond postestimation, [XT] xtdpdsys postestimation, and [XT] xtdpd
postestimation.
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