A method for evaluating and modifying the operating conditions of a machine or an installation includes measuring at least one parameter a number of times to create a database, of the parameter, determining an extrapolation range, performing an extrapolation in the range, and determining a measure of quality, K, of the extrapolation calculated on the basis of the database. K is a function of at least two variables. The parameter value is controlled so that the value will be retained within a limit.