The Field Representative's Manual states, "Mental Retardation refers to someone who is significantly below average in intellectual functioning, in addition to having problems with adaptive behavior." This official definition was not routinely quoted as part of the interview process and presumably would have been cited only if a subject asked for clarification.

In 1994 and 1995, this variable is part of the Disability Supplement (NHIS-D) 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.

Information was collected separately on ADD/ADHD (Attention Deficit Disorder/Attention Deficit Hyperactive Disorder) (ADDEV) for sample children aged 2 to 17 and on "Other Developmental Delay" (ODDEV) for all sample children. Prior to 2022, information was collected on one of the main causes of mental retardation, Down Syndrome (DOWNSYNEV). To see if the sample person currently has an intellectual disability, see INTDSNW.

Intellectual disability, also known as mental retardation was also incorporated into the survey as one of the possible conditions or health problems that respondents could select from a list on a flash card, to explain why individuals were limited in their activities. Starting 2019, flash cards were eliminated.