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Introduction
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Definition and classification of learning disability
Must fulfil 3 criteria:
- Intellectual impairment (IQ </= 70)
- Social or adaptive dysfunction
- Onset in the developmental period (thus excluding people with dementia or other adult-onset diseases/injury affecting the brain)
Learning disability is a descriptive diagnosis or concept, not a disease or illness. It does not infer a particular aetiology. Social functioning is an integral part of the diagnosis. It is important to understand that it is different from mental illness - a person with a learning disability can also develop mental illness. Learning disability as a concept is also different from ‘learning difficulties', which generally refers to specific learning problems (e.g. dyslexia), rather than a global impairment of intellect and function.
IQ (Intelligence Quotient)
IQ is used to classify level of learning disability
Mild learning disability |
50 to 70 |
Moderate learning disability |
35 to 50 |
Severe learning disability |
20 to 35 |
Profound learning disability |
<20 |
Note that IQ of 70 is 2 standard deviations below the mean, and is therefore considered statistically significant.
IQ may be an easily quantifiable way to classify learning disability, but there are problems with it. For example, the overall IQ score does not indicate individual strengths or weaknesses (e.g. verbal and motor skills). IQ also varies during development. IQ classification alone does not include the important area of social adaptation and functioning.
Prevalence
Statistically the prevalence of people with IQ<70 should be 2.5% (2SD from mean). Actually the prevalence of people with learning disability is 1-2%, because of:
- Differential mortality (the more severe the degree of learning disability, the higher the mortality c.f. general population).
- Diagnostic changes with time, not all cases classified.
- The role of functioning - those with IQ<70 but no problems functioning within their environment would not be defined as having a learning disability.
Prevalence of people with IQ <50 is 0.35%.
IQ levels follow a normal distribution curve for IQs above 70. Below this the curve has an asymmetric distribution, with higher than expected numbers at IQ's of less than 70 and a small ‘bump' at the lower end. The normal distribution represents the differences in IQ secondary to environmental and genetic effects right across the IQ range. The bump at the lower end is due to organic causes.
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