Method in an oil and/or a gas production system including 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. 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. The method includes the steps of 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, determining fitted model parameters so that predictions of produced fluids match said historical production measurements as closely as possible, and determining a quality tag that describes the uncertainty of the predictions of change in produced fluids.