noconstant suppresses the constant term in the levels equation.  By default, the term is included as a regressor and IV-style instrument.  Unlike xtabond and dynamic panel data (DPD) (the original implementation of these estimators), xtabond2 does not include the constant term in the transformed equation in difference GMM; rather, the constant is transformed out.
差分方程本来就不会有常数项啊？为什么xtabond和xtdpdsys默认包含常数项呢？
不加noc选项是否常常意味着模型设定错误？