Introduction In many fields of economics researchers have been forced to test their theoretical models of individual behaviour on aggregate data because of lack of individual data. However, the aggregation level of data can have a strong influence on the results of empirical econometric research. It is not surprising then that the theoretical problems associated with aggregation have received a great deal of attention among econometricians (see, e.g. Theil, 1954, Green, 1964, or Van Daal and Merkies, 1984).