In model-building investigations the response of interestyis often a function of two or more components. The usual procedure in such situations is to use this function to compute the values ofyfor each run and then obtain the fitted equation ŷ =F(x1,x2, …,xk) relating ŷ to relevant process variables. In this article we suggest a different method that will sometimes be preferable: first fit equations to the component responses (either individually or in subsets) and then combine these equations to obtain the desired functionF. The proposed method is especially useful when the usual procedure fails to produce a satisfactory model. A chemical example illustrates how, by using this method, a second-order model can be obtained from a first-order design (a two-level factorial design).