Reference person.The person around whom the family relationship of all the other members of the family is defined. In the case of nuclear families, the guiding principle says that the reference person should be the father, or, in his absence, the mother. In multi-nuclear families the reference person should be the father, or, in his absence the mother, of the eldest generation or, within the same generation, the eldest person. If there is no family nucleus, whether the people are related or otherwise, the reference person is given to be the eldest person. For each family there can be one reference person only.

Head of group: As above, the person who best defines the family relationships within the nuclear family; this means the father or the mother. The reference person is the head of their own nucleus, should they be included in one.

Members of the nucleus: The remaining people in each family nucleus.

People not belonging to the nucleus: Part of the group of isolated population in the family, which also include reference persons in non-nuclear families.