Understanding the prevalence of developmental disability (DD) is important for evaluating risk and protective factors and appropriate supports and services for optimal outcomes among children. The term “developmental disability” is defined and applied in different ways depending on purpose and across sources, including in federal and state policies and laws.1, 2, 3, 4, 5 Methodological clarity is crucial for appropriately addressing public health questions; the definitions used in surveillance and research shape who may be included in data, and thus should be aligned with the intended use of the data. Without understanding the impact of how DD is defined, comparisons across data sources and analyses may not be meaningful.

The term “disability” itself lacks a singular definition.6 The International Classification of Functioning, Disability, and Health defines disability as “an umbrella term for impairments, activity limitations, and participation restrictions”.7 While this paradigm is widely accepted, the concept is operationalized in different ways, most based on either functional limitations or identified diagnoses. The traditional medical model of disability focuses on a professionally-designated diagnosis, which does not always correlate with uniform functional limitations. For example, one child with autism spectrum disorder (ASD) may be minimally verbal whereas another child with the same diagnosis may be highly verbal with the ability to use complex sentences. On the other hand, disability defined by functional limitations does not rely on an underlying medical diagnosis. For example, a child with a mobility limitation could have a variety of different underlying diagnoses such as cerebral palsy, muscular dystrophy, or an injury. When assessing interventional needs and health equity issues within the community, diagnosis may be less important than how the environment in which someone lives either impedes or improves their ability to function. This latter definition of disability may be more useful for identifying, developing, implementing, and evaluating supports, and is in line with eligibility determinations for services such as special education supports based on functioning in the educational setting.8 However, knowledge of a diagnosis that is leading to functional limitations may help inform treatment and prevention, so there may often be utility in capturing both functional and diagnostic information.

The addition of the word “developmental” to “disability” compounds the challenge of measurement. DDs are a group of conditions due to an impairment in physical, learning, language, or behavior areas that begin during the developmental period and usually last throughout life.9 However, there is variation in the number, types, and degree of impairments to be considered DDs. For example, the Developmental Disabilities Assistance and Bill of Rights Act of 2000 (DD Act) indicates that in addition to being due to a physical or mental condition manifested before age 22 and expected to continue indefinitely, the person should have substantial functional limitations in 3 or more areas of 7 major life activities and need specialized services and supports for an extended duration, if not for a lifetime.2 The DD Act definition tends to focus on individuals with higher degrees of disability requiring more services and supports. However, children with specific developmental disabilities such as a speech-language impairment or learning disability may still require and benefit from specialized supports but may not qualify as having a DD depending on the context. Also, while the age range encompassing the developmental period is defined as through 18 or 22 years of age, it is less clear whether disability due to injury or illness with later onset is considered a DD. Public health researchers often leverage available data sources that only provide information on reported or documented diagnoses, which does not provide specific information on functional limitations. However, more national surveys have begun measuring disability using questions targeted at categorizing functional limitations.

Recent work highlights the difficulty of measuring disability and DD in surveys. Hagerman et al. evaluated the prevalence of disability based on existing definitions using a single data source, the National Survey of Children's Health (NSCH).10 They found only a small degree of overlap across the definitions; 23.5% of U.S. children in the sample met one or more definition and only 8.7% of children met all five definitions. Anderson et al. performed a systematic review of studies reporting the prevalence of intellectual or DDs in all ages within the United States since 2000, finding that prevalence varied across years, ages, definitions, and data sources.1 These results are unsurprising given the complexities in defining disability, DD, and functional limitations, particularly during the dynamic periods of childhood and adolescence.

This paper builds upon past work by examining multiple definitions of disability and DD in children, comparing prevalence based on those definitions across multiple national surveys, and using more recent data sources. Our aim was to investigate how different definitions of developmental disabilities impact the estimated prevalence using multiple national, publicly available surveys. We anticipated the best alignment to be among definitions using similar scope of definition (e.g., disability or developmental disability) and data collection purposes (e.g., domains of functional disability or identified diagnoses/conditions).