Journal Issue: Special Education for Students with Disabilities Volume 6 Number 1 Spring 1996
Distribution and Severity of Disabilities
In understanding the distribution and severity of disabilities, it is important to remember that (1) the prevalence of disabilities varies by age and category, (2) the high-incidence disabilities such as learning disability and speech or language impairment occur primarily at the mild level, and (3) even "mild" disabilities may constitute formidable barriers to attaining adult goals such as a high-status career.
From the summary of disabilities by category for children ages 6-11 and 12-17 provided in Table 2, several trends are apparent. Learning disability is the most frequently occurring disability at both age intervals, a trend that is particularly prominent at the 12-17 age interval. The prevalence of speech and language disabilities declines substantially with increasing age. Also, although there are 13 categories, more than 90% of the children classified as disabled in school settings are accounted for by learning disability, speech or language impairment, mental retardation, and serious emotional disturbance. (See the Child Indicators article by Lewit and Baker in this journal issue for a discussion of changes in the prevalence of learning disability and mental retardation since the inception of Public Law 94-142.)
The severity of disabilities also varies within categories. Severity is influenced by (1) the size of the deficit in behavior or skills; (2) the number of areas in which there are deficits; and (3) the amount and kind of support needed to participate in daily activities such as learning, work, leisure, self-care, and mobility in the community. Persons with disabilities at a severe level typically have large deficits, often in two or more areas, that require extensive and consistent support. Persons with disabilities at the mild level typically have smaller deficits on the key dimensions, deficits in fewer areas, and can function without assistance in most of the normal daily activities.
Knowledge of the exact distribution of severity within disability categories is extremely limited. In broad terms, however, it appears that the majority of students diagnosed with learning disability and speech or language impairment have disabilities at the mild level. The level of disabilities in mental retardation and serious emotional disturbance can vary from mild to severe; however, at least half are at the mild level.17,18
As noted earlier, the distinction between disability and normal ability/behavior is somewhat arbitrary and subject to local preferences. In particular, the mild disabilities exist on broad continua in which there are no clear demarcations between those who have and those who do not have the disability. Yet, special education eligibility is a dichotomous decision: the student either is or is not eligible for services. In many states, a point or two on discrepancy scores (intended to measure the discrepancy between a student's ability and achievement) can determine whether or not several thousand additional dollars are spent on the child's education. Such momentous decisions are not supported by our knowledge of the distribution curve. One of the key findings in the National Institute of Child Health and Human Development (NICHD)-funded studies on learning disabilities (see the article by Lyon in this journal issue) involves the impossibility of clearly differentiating between dyslexia (a common learning disability) and low achievement in reading: "This study allowed us to investigate the commonly held belief that dyslexia is a discrete diagnostic entity. Our data do not support this notion. Rather, they suggest that dyslexia occurs along a continuum that blends imperceptibly with normal reading ability. These results indicate that no distinct cutoff point exists to clearly distinguish children with dyslexia from children with normal reading ability; rather, the dyslexic children simply represent the lower portion of the continuum of reading capabilities."19
Finally, the generalizations that a disability such as learning disability nearly always is mild and that, as adults, persons with learning disability usually are not officially recognized as disabled does not mean that mild disabilities are trivial or that they magically disappear at age 18 or 21. In fact, students with learning disability are seriously impaired in one of the most important developmental tasks in a technologically complex society: acquiring literacy skills and using those skills to master bodies of knowledge. Poor reading skills in particular constitute formidable barriers to both education and occupational attainment and significantly limit adult career opportunities (see the article by Wagner in this journal issue).