1. A method for adjusting a chemical dosage of a pulp processing stage, the method comprising determining the value of a variable in dependence on the chemical dosage prior to adding the chemical dosage, measuring the chemical dosage to be added, determining the value of the variable after adding the chemical dosage, determining, on the basis of the above-mentioned stages, the change in the variable caused by the chemical dosage, determining a model describing the change in the variable as a function of the chemical dosage, determining a performance index for the model, comparing the performance index of the model with the performance indices of certain previously determined models, bringing into use the model that produced the best performance index in the comparison, and determining the necessary chemical dosage by means of the model put to use.