最近投了一篇论文，外审专家返回了一个修改意见，看不懂，请各位大神帮忙。

System GMM requires the additional assumption that the differences used as instruments are uncorrelated with the error term (in particular with the unobserved unit-specific effects). A sufficient condition for this to hold would be joint mean stationarity of the dependent variable and the independent variables, which may not be easily justifiable in this study.


翻译过来是这样：系统GMM需要额外的假设，即作为工具使用的差异与误差项不相关(特别是与未观测到的特定单位效应无关)。一个充分条件是因变量和自变量的联合平均平稳性，这在本研究中可能不容易证明。


请问这个问题怎么解决呢？求大神帮忙