System GMM
Estimate dynamic panels through defensible moment conditions
System GMM combines differenced and level equations for dynamic panels, using lag-based instruments justified by moment restrictions. The level equations add restrictions on initial conditions beyond those used by difference GMM. It is commonly considered for many entities and relatively few periods, but instrument proliferation, weak instruments and invalid exogeneity classifications can make apparently precise results unreliable.
Choose this for dynamic panels with a lagged outcome and potentially endogenous regressors when credible internal moment conditions can be defended. The panel structure and instrument strength must suit the proposed estimator.
Strengths
- Handles entity effects and lagged dependent variables under explicit moments
- Additional level moments can improve weak difference-GMM performance
Limitations
- Extra moments require substantive initial-condition restrictions
- Too many instruments can overfit endogenous variables and weaken specification tests
Know the boundary
System GMM does not automatically solve endogeneity; the extra level moments can fail even when difference-GMM moments are valid.