The importance of cooperative interaction has been pointed out in the spin crossover phenomena. Sorai and co-workers have proposed a so-called domain model, in which the degree of correlation is expressed by a phenomenological parameter characterizing the sharpness of the transition. Wajnflasz has introduced a pioneering statistical model, in which the microscopic interaction and the entropy effect are considered. In the present work, we study the relation between these two models. In particular, we derive the analytical relation between the effective interaction range of the domain model and the microscopic parameters of the Wajnflasz model. (C) 2003 American Institute of Physics.