2. The system has a direct term (relative degree zero). Then it is essential to use nk = 0 for these inputs in all model

[fyousefi]

2. The system has a direct term (relative degree zero). Then it is essential to use nk = 0 for these inputs in all models. (Note that state-space models have nk = 1 as default, so use: MODEL = PEM(Data,n,''nk' ), where n is the model order.)