1. A method comprising the steps of: developing a mathematical model of a response surface as a function of the values of a plurality of input variables; determining an actual value of a response variable at a nominal value of a first input variable; measuring an actual value of the first input variable; using the mathematical model to calculate a first modeled value of the response variable based on the measured actual value of the first input variable; using the mathematical model to calculate a second modeled value of the response variable based on the nominal value of the first input variable; calculating an estimate of the response variable by adding the actual value of the response variable measured at the nominal value of the first input variable to the first modeled value of the response variable and subtracting the second modeled value of the response variable; and controlling an output device based on the calculated estimate of the response variable.