The Field Representative's Manual for 2001 forward defined Down Syndrome as "a congenital disorder characterized by moderate to severe mental retardation, slow physical development, and flattish skull and facial features." This definition was not routinely shared with respondents.

A related question asking whether the person has mental retardation (RETEV) is available for the same years as DOWNSYNEV.

This variable is part of the Disability Supplement (NHIS-D) in 1994 and 1995 that collects information on disability. NHIS-D included two household interviews: in Phase I all members of sample families were screened for any indication of disability; in Phase II persons with any indication of disability were followed-up for additional information. Phase I includes sensory, communication, and mobility problems; health conditions; activities of daily living and independent activities of daily living; functional limitations; mental health; services and benefits; special health needs of children; early child development; education; relationship to respondent; and perceived disability. Phase II includes four Disability Followback Survey (DFS) questionnaires: one for children, one for adults, one for elderly persons (69 years of age and over) without any indication of disability (also called the Supplement on Aging or SOA; only in 1994), and one for persons with a history of polio.