Hi, in the System Identification Toolbox nonlinear grey-box model parameters are identified using the Prediction Error Method. I would like to know in this algorithm the prediction step works. nlgrey models are defined by a set of nonlinear ODEs and a set of output equations. This means that for a prediction, an internal state has to be estimated from the measurements given up to this point. Is this done using an observer? If so, what kind of observer is used and how is (partial) un-observability handled? If not, is the prediction step just an open-loop simulation of the system without feedback of the prior measurement data?