The Index Person—Relation to Incidence Rates in Family Studies

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Abstract
In investigations where the family is the unit of study, the index person is the member responsible for bringing the rest of the family into the study. If incidence rates for some event are to be set up on the family experience and compared with those in the general population, it is critical to determine whether the event causing selection of the index person is in any way related to the event under study. If there is any relationship whatsoever between the selection of the index person and the event for which the rate is set up, then the index person belongs to a different universe of discussion from that of his familial associates and should be excluded from the calculations. This relationship may be positive; that is, the index person may certainly have or be an increased risk of having the event being studied. This case is illustrated by studies of morbidity or mortality of tuberculosis where the index person is a known case. The association may be negative where the index person has no risk or a decreased risk of the event in question. This is illustrated by the retrospective study of mortality or morbidity in family groups where the survival of the index person is essential to the inclusion of the family in the study. In all such cases the experience of the index person and that of his family associates cannot be combined to produce meaningful rates. In the rather rare family studies where the choosing of the index person is entirely unrelated to the event under study, and where only the family experience subsequent to the time of selection of the family is considered, the index person and his family associates may be considered as belonging to the same universe of discussion, and incidence rates for their combined experience may be set up.

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