xtabond2 may hold the record among user-written Stata modules for the most confused users (and perhaps the most-confused too). In this presentation, I motivate and describe, in pedagogic fashion, the Arellano-Bond and Blundell-Bond linear GMM dynamic panel estimators, drawing lessons from a steady stream of correspondence with users. And I provide an overview of how to implement them with xtabond2. I first introduce linear GMM as an extension of OLS. Then I describe how limited time span, the potential for fixed effects, endogeneity, and the dangers of dynamic panel bias all shape these estimators—in particular, in their use of differences, lags as instruments, and GMM. I explain how xtabond2 commands should be constructed, with particular attention to the various options and sub-options for controlling instrument matrix construction. I discuss the need to limit the number of instruments and options for doing so.
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