More evidence on the behaviour of the GMM System estimates under the misspecified AR1:
The GMM System seems to select the highest AR parameter in the groups of data as its estimate for the misspecified AR parameter. The GMM Difference is usually a little lower in its estimates, but still close to the top end of the range. Arellano-Bond and OLS are usually around the GMM Dif level. It is probable that the OLS estimates are the product of two factors - a downward bias in estimates because of the OLS estimate of a correctly specified AR1, then a selection of a high parameter among the set of estimates.
The results are robust to the small changes in GMM estimation method between the software packages DPD for Ox and Stata.
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