ADDITIVE AND NONADDITIVE EFFECTS OF 2 FACTORS ON A RESPONSE FUNCTION

General dependence of a response function on two factors can be always approximated by a linear and additive empirical relationship. When this is not sufficiently precise, a nonadditive relationship is preferred which is nevertheless linear in each variable separately. Properties of this equation are shown in examples, possible interpretation of the parameters is discussed, and the characteristic phi is proposed, measuring quantitatively the deviation from additivity. For distinguishing additive and nonadditive behavior of a certain response function, several statistical methods were tested, and a method was described which avoids any external parameters and is more efficient than a method proposed recently. There is good evidence that nonadditive relationships of this type can be used much more extensively than it has been usual in the literature. Extension to a function of three factors was briefly examined.