On the assessment of geographical survey units using constrained classification

Surveys of spatially distributed phenomena are often conducted using geographical areas as strata. If CATI (computer assisted telephone interviewing) methodology is used to contact the units, it is possible to choose between two different methods of selecting the units to be interviewed: either from a full list of the population units or a selection based on RDD (random digit dialling) technique. On this last case it seems natural to telephone exchange areas as strata. This could be a very interesting solution from many point of views. The aim of this paper is to find a methodology to asses the opportunity of using such a pre-defined geographical stratification in comparison of the usual clustering methods, based on a set of auxiliary variables correlated with the phenomena under study, to define the strata. The choice will depend on the use of some measure of similarity and/or the evaluation of the homogeneity in the strata for the specific phenomenon to be analysed (in our application the evaluation of the loss of homogeneity is verified with respect to a hypothetical set of variables under study).