Mobility Effect Models: A Reply to Schumm et al.

The article presents a reply to Walter R. Schumm and his colleagues, who have proposed an alternative means of dealing with situations involving an hypothesized effect of the difference between two variables. Their approach stresses the role of theory in specifying a priori an appropriate three-dimensional geometric model. They proposed that what they consider an alternative means of dealing with situations involving an hypothesized effect of the difference between two variables. Their approach stresses the role of theory in specifying a priori an appropriate three-dimensional geometric model and the testing of this model against alternative plausible models using regression. In the earlier article Charles J. Brody and James A. McRae made use of previous theory and research on the effects of social mobility to select an appropriate dependent variable for analysis, to propose two types of identifying restrictions leading to plausible three-dimensional models, and to rule out other identifying restrictions leading to less plausible models or unreasonable constraints on the mobility parameters.