Although it is encouraging that no individual who was offered participation in this study refused to participate themselves, only one fifth of the probands agreed, during initial evaluations, to provide permission for research staff to directly contact other members of their families. Individuals who agree to contact their own family members do so at rates substantially lower than the contact rates achieved when family contacts are initiated by research staff members. Such differential participation may well provide occult differences between samples collected for association vs linkage studies.
