The average number of children expected and the average difference between the number of children expected and of men between 15 and 40 years of age per generation.

The count and description of the families of the Basque Country is carried out using various statistical sources. On the one hand, family counts are carried out through the Population and Housing Census; on the other hand, the Demographic Survey (ED) operation, of an intercensal nature and of five-year periodicity, provides information on the formation of families, family size, preferences on the number and spacing of children, as well as numerous other characteristics, using the method of retrospective approach to demographic phenomena.