“…However, two major limitations of this approach are that: (i) it is based on the restrictive assumption that individual inputs are non-specific to individual outputs so that no input has a comparative advantage in the production of any particular output (McKay et al, 1983); and (ii) apart from being a trivial case, it forces production to be a joint supply process, not just a multiple-output process (Livernois and Ryan, 1989). The alternative approach, which allows for flexible production relationships, is based neatly on the theory of duality whereby a second-order approximation to the true (although unspecified) profit function is specified and estimated (McKay et al, 1982(McKay et al, , 1983Wall and Fisher, 1987;Lawrence and Zeitsch, 1989;Fisher and Wall, 1990;Coelli, 1996). In the latter case, the flexible functional forms commonly chosen to specify the joint production in Australian agriculture include normalized quadratic (proposed by Lau, 1974Lau, , 1976a; also see Diewert and Wales, 1987) and transcendental logarithmic (propounded by Christensen et al, 1973;Diewert, 1974) function.…”