1. A method in an oil and/or a gas production system comprising a plurality of oil and/or gas wells each producing a multiphase fluid stream, adapted for predicting change in produced fluids resulting from change in manipulated variables, characterized in that fitted model parameters which express the relationship between the change in manipulated variables and the produced fluids are determined from a set of historical production measurements, said method comprising the steps of a. choosing a model structure which predicts change in produced fluids as a function of the change in manipulated variables, where the predicted change in produced fluids depends on the value of fitted model parameters b. determining fitted model parameters so that predictions of produced fluids match said historical production measurements as closely as possible c. determining a quality tag that describes the uncertainty of said predictions of change in produced fluids.