Polynomial conjoint measurement is an extension of the theory of conjoint measurement to three or more attributes. It was initially developed by the mathematical psychologists David Krantz and Amos Tversky. The theory was given a comprehensive mathematical exposition in the first volume of Foundations of Measurement, which Krantz and Tversky wrote in collaboration with the mathematical psychologist R. Duncan Luce and philosopher Patrick Suppes. Krantz & Tversky also published a non-technical paper on polynomial conjoint measurement for behavioural scientists in the journal Psychological Review. As with the theory of conjoint measurement, the significance of polynomial conjoint measurement lies in the quantification of natural attributes in the absence of concatenation operations. Polynomial conjoint measurement differs from the two attribute case discovered by Luce & Tukey in that more complex composition rules are involved.
Polynomial conjoint measurement
Krantz's (1968) schema
Most scientific theories involve more than just two attributes; and thus the two variable case of conjoint measurement has rather limited scope. Moreover, contrary to the theory of n – component conjoint measurement, many attributes are non-additive compositions of other attributes. Krantz proposed a general schema to ascertain the sufficient set of cancellation axioms for a class of polynomial combination rules he called simple polynomials. The formal definition of this schema given by Krantz, et al., is as follows. Let. The set is the smallest set of simple polynomials such that:
;
such that and, then and are in.
Informally, the schema argues: a) single attributes are simple polynomials; b) if G1 and G2 are simple polynomials that are disjoint, then G1 + G2 and G1G2 are simple polynomials; and c) no polynomials are simple except as given by a) and b). Let A, P and U be single disjoint attributes. From Krantz’s schema it follows that four classes of simple polynomials in three variables exist which contain a total of eight simple polynomials:
Additive: ;
Distributive: ; plus 2 others obtained by interchanging A, P and U;
Krantz’s schema can be used to construct simple polynomials of greater numbers of attributes. For example, if D is a single variable disjoint to A, B, and C then three classes of simple polynomials in four variables are A + B + C + D, D + and D + ABC. This procedure can be employed for any finite number of variables. A simple test is that a simple polynomial can be ‘split’ into either a product or sum of two smaller, disjoint simple polynomials. These polynomials can be further ‘split’ until single variables are obtained. An expression not amenable to ‘splitting’ in this manner is not a simple polynomial.
SINGLE CANCELLATION. The relation " " satisfies single cancellation upon A whenever if and only if holds for all and. Single cancellation upon P and U is similarly defined.
DOUBLE CANCELLATION. The relation " " upon satisfies double cancellation if and only if for all and, and therefore is true for all. The condition holds similarly upon and.
JOINT SINGLE CANCELLATION. The relation " " upon satisfies joint single cancellation such that if and only if is true for all and. Joint independence is similarly defined for and.
DISTRIBUTIVE CANCELLATION. Distributive cancellation holds upon if and only if, and implies is true for all and.
DUAL DISTRIBUTIVE CANCELLATION. Dual distributive cancellation holds upon if and only if
,, and implies is true for all and.
SOLVABILITY. The relation " " upon is solvable if and only if for all and, there exists and such that.
ARCHIMEDEAN CONDITION.
Representation theorems
The quadruple falls into one class of three variable simple polynomials by virtue of the joint single cancellation axiom.
