I am implementing system GMM using the xtabond2 command. The data series is very long. This long history generates a very high number of instrumental variables (IV) which creates a significantly over identified result. Unfortunately, when I include the collapse option, the results change from what is observed in previous literature and don’t make intuitive sense. I suspect the averaging of the individual instruments across all the observations is reducing the information necessary to produce meaningful coefficients. Do you know how to modify the collapse option so that the IV matrix only collapses the earlier observations of the instrument matrix and leaves the more recent observations? Thank you, Dan