NHIS-D Studies

Defining Mental Retardation and Developmental Disabilities

The NHIS-D permits examination of the implications of various definitions of mental retardation and/or developmental disabilities on prevalence estimates, service eligibility, and other topics requiring large and flexible data sets. To examine the nature and congruence of prevalence estimates for mental retardation and for developmental disabilities as defined in the DD Act, investigators from the Research and Training Center on Community Living at the University of Minnesota developed a multi-stage process for operationalizing both mental retardation and developmental disabilities using data elements contained in the 1994/1995 NHIS-D. This process included reviewing the definitions of these constructs used in planning and developing the NHIS-D and its instruments; preparing and revising initial operational definitions using the NHIS-D data elements based on reviews solicited from a panel of national experts in mental retardation and developmental disabilities diagnosis and research; and reviews and revisions of specific codes based on internal validation analyses and internal and external reviews of results. The final definitions are described in the following paragraphs.

Identifying people with mental retardation

People with mental retardation were identified in one or more of the following ways --

• People were identified as having mental retardation if the household respondent answered affirmatively to a question asking whether anyone in the household had mental retardation.

• People were also identified as having mental retardation when mental retardation was indicated as the cause of age-specific general activity limitations. General activity limitations included limitations in play for children ages 5 and younger, limitations in school activities for children ages 5 to 17, limitations in work for adults ages 18 to 69, and overall limitations in activities for persons of all ages.

• People were identified as having mental retardation if mental retardation was identified as the primary cause of limitations in communication, getting along with others, activities of daily living, instrumental activities of daily living, and other functional limitations; or if mental retardation was the ICD code listed as the reason the person had a doctor’s visit, a physician consultation regarding communication, or as the reason for receiving occupational therapy.

People were also identified as having mental retardation through a two-step process involving individuals who reported having a condition frequently associated with mental retardation. The first step identified people if the household respondent answered affirmatively to a question asking whether anyone in the household had autism, cerebral palsy, Down syndrome, spina bifida, or hydrocephalus, or who reported having one or more of the conditions listed in the table below as the cause of either age-specific general activity limitations in the Core Survey, as the cause of specific activity limitations (e.g., communicating, getting along with others) or as the reason for receiving various services (e.g., occupational or physical therapy) in the Phase 1 Disability Supplement.

Individuals meeting these criteria were identified as having related conditions. In the second step, individuals with a related condition were screened to identify people who also reported having both a learning disability (as reported by the household respondent when asked if anyone in the household had a learning disability) and a significant functional limitation in learning.

A significant functional learning limitation was ascribed to children ages 1 to 17 years who had problems or delays in understanding things, that is, delays in cognitive or mental development with the problem having been mentioned by a physician or other health care professional. A significant functional learning limitation was also ascribed to individuals ages 5 or older who had serious difficulty learning how to do things that most people their age are able to learn. Individuals who had one or more related conditions who also reported having a learning disability and a significant functional learning limitation were considered to have mental retardation for this analysis. However, children ages 5-17 with related conditions were not included in the mental retardation group based on their related condition if their learning difficulty was not serious enough to require special education or to limit or prevent school attendance.

Adults with related conditions were not included in the mental retardation group based on their related condition if they completed more than one year of post-secondary education. Furthermore, adults with related conditions were not included in the mental retardation group based on their related condition if they had Alzheimer’s disease because that condition may have been the cause of their learning limitations.

Overall, 871 people were identified as having mental retardation within the 1994 NHIS-D sample and 740 people were identified as having mental retardation within the 1995 NHIS-D sample. Of these people, 116 sample members in 1994 and 106 in 1995 were identified as having mental retardation based solely on having a related condition along with a learning disability and a significant functional learning limitation.

Definition of Developmental Disabilities

According to the Developmental Disabilities Assistance and Bill of Rights Act (DD Act) Amendments of 1996, a developmental disability is a severe, chronic disability of an individual 5 years of age or older that --

• is attributable to a mental or physical impairment or combination of mental and physical impairments;
• is manifested before the individual attains age 22;
• is likely to continue indefinitely;
• results in substantial functional limitations in three or more of the following areas of major life activity: self-care; receptive and expressive language; learning; mobility; self-direction; capacity for independent living; and economic self-sufficiency; and
  reflects the individual’s need for a combination and sequence of special, interdisciplinary, or generic services, supports, or other assistance that are of lifelong or extended duration and are individually planned and coordinated
• except that such a term, when applied to infants and young children, means individuals from birth to age 5, inclusive, who have substantial developmental delay or specific congenital or acquired conditions with a high probability of resulting in developmental disabilities if services are not provided (PL 103-120).

A multi-stage process was used to identify individuals with developmental disabilities. The process involved using NHIS-D variables to construct operational definitions for each of the seven areas of functional limitation in the federal developmental disabilities definition:

1. self-care,
2. expressive or receptive language,
3. learning,
4. mobility,
5. self-direction,
6. capacity for independent living, and
7. economic self-sufficiency.

The initial operational definitions were revised following review by a panel of experts in disability research. As conveyed in the DD Act, separate definitional approaches were required for persons 5 years and older and for children from birth to 5 years. In the following summary of the final operational definitions of functional limitations, actual NHIS-D item language is enclosed in quotation marks. In each of the seven areas of major life activity, to meet our criteria, limitations must have been first experienced before age 22 and must have been expected to endure for at least 12 months. This latter expectation was the best available proxy for the DD Act criterion of “is likely to continue indefinitely.” In general, the age limitations noted here reflect the ages of the people who were asked to respond to particular questions on the NHIS-D.

Self-care

• A person 5 years or older “has a lot of difficulty” or “is unable” to dress, eat, bathe, get in and out of bed or chairs, use the toilet, or get around the house.

Expressive or receptive language

• A person 5 years or older “has serious difficulty communicating so the family can understand” or “has serious difficulty understanding others when they talk or ask questions.”
• A person 18 years or older “has serious difficulty” or “cannot use” the telephone.
• A person from 5 to 17 years old “has a problem or delay in speech or language development” mentioned by a doctor and these limitations were serious enough to require special education or to prevent or limit school attendance; or “has significant problems at school communicating with teachers and other students” or “cannot communicate with teachers and other students because of limitations” and these limitations were serious enough to require special education or to prevent or limit school attendance.

Learning

• A person 5 years or older has a diagnosis of mental retardation or autism, and “has serious difficulty learning how to do things most people their age can learn.”
• A person 5 years or older “has serious difficulty learning how to do things most people their age can learn” and the person does not have Alzheimer’s or another senility disorder.
• A person from 5 to 17 years old “has problems or delays in understanding things, that is, delays in cognitive or mental development” that are serious enough to be mentioned by a doctor.
• A person from 5 to 17 years old “has significant problems at school understanding instructional materials” or has been diagnosed as having learning disability and the person has received special education, has had an IEP or has been limited in school attendance.
• An adult ages 18 or older has been diagnosed as having a learning disability significant enough that the person’s overall educational background has included less than 2 years of post-secondary education.

Mobility

• A person 5 years or older “has difficulty” or “is unable” to walk up stairs, walk three city blocks, or transfer to or from a bed or chair.
• A person 5 to 17 years old “has difficulty participating in strenuous activity compared to other children his/her age” (and it is associated with arthritis, rheumatism, disorders of the bone or cartilage, absence or paralysis of an extremity or orthopedic impairment or deformity) or “has significant problems in physical development.”

Self-direction

• A person 5 years or older, because of a physical, mental or emotional problem, “needs to be reminded or have someone close by” for dressing, eating, bathing, toileting, or transferring.
• A person 18 years or older has or needs a “case manager to coordinate personal care, social or medical services” or “has a court appointed guardian” during the last 12 months.
• A person 5 to 17 years old “has significant problems with” or “cannot pay attention in class” or “control behavior in class” due to a limitation and these problems were severe enough that the child has received special education, has had an IEP, or has been limited in school attendance.
  Capacity for independent living
• A person 18 years or older “requires help or supervision” or “has a lot of difficulty with” or “is unable” to prepare meals, shop for personal items or medicine, manage his or her money, do light work around the house (such as doing dishes, straightening up, light cleaning or taking out the trash) or do heavy work around the house.

Economic self-sufficiency

• A person 18 years or older “has never been able to work” or “is currently unable to work because of a mental or emotional problem” or “is limited in kind or amount of work” due to a limitation; or “has trouble finding or keeping a job or doing job tasks because of a mental or emotional problem.”

• A person 18 years or older “has participated in” or is “on the waiting list” for a sheltered workshop, transitional work training, supported employment or a day activity center; or “is unable to work” because of a mental or emotional problem.

Determination of Developmental Disability

People 5 years and older were considered to have a developmental disability if they had substantial limitations as defined above in three or more of the seven areas of major life activity that were expected to endure at least 12 months, with limitations in at least one of these areas occurring before age 22.
Based on the definition in the DD Act, children from birth to 71 months old were considered to have a developmental disability if they had a specific congenital or acquired condition that was consistently associated with developmental disabilities among older children and adults in the NHIS sample, or a substantial developmental delay in physical development, chewing, eating or toileting, receptive or expressive communication, understanding instructional materials, cognitive or mental development, participation in strenuous activity, or mobility. Children ages 5 and younger who had only a hearing, vision or speech impairment were not considered to have a developmental disability.

Overall, 1,207 people of all ages in the 1994 sample and 1,118 people in the 1995 sample met the criteria for having a developmental disability. Of those people, 464 in 1994 and 396 in 1995 also met the criteria for having mental retardation. A total of 1,614 sample members in 1994 and 1,462 sample members in 1995 met the criteria for either mental retardation and/or developmental disabilities. An additional 770 people in 1994 and 840 people in 1995 were identified as having cerebral palsy, autism, spina bifida, epilepsy or one of the other related conditions, but did not meet the criteria for either mental retardation or developmental disabilities as defined for these analyses.

This research is funded by The National Institute on Disability and Rehabilitation Research (NIDRR), U.S. Department of Education, through a Cooperative Agreement (No. H133A60051) with The Center on Emergent Disability, University of Illinois at Chicago; through a NIDRR Field-Initiated Grant (No. H133G80082) to The Research and Training Center on Community Living at the University of Minnesota; and through support of the RISP project provided by the Administration on Developmental Disabilities (Grant No. 90DN0028/01). Funding for this publication is provided through a NIDRR Cooperative Agreement (No. H133B980047) with the Research and Training Center on Community Living, University of Minnesota.