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University of Guelph
PSYC 3850

The Epidemiology of Mental Retardation Challenges and Opportunities in the New Millennium Helen Leonard and Xingyan WenIntroduction y The definition classification and measurement of mental retardation MR have involved considerable controversy over time y Some would argue that the lumping together of so many different underlying disorders and pathological processes within a single entity should not even be considered y The name assigned to this group of conditions varies internationally y Epidemiology has been defined as the study of the distribution and determinants of healthrelated states or events in specified populations and the application of the study to the control of health problems Last 1995 y Applying the conventional principles of epidemiology to the study of MR helps examine how common MR is in the community whether incidence or prevalence has changed over time and whether certain population subgroups are more or less likely to be affected y Epidemiological methods help identify those casual determinants where prevention is possible and then whether the appropriate intervention has had an impact on prevalencey Epidemiologists can evaluate quality of life and access to medical and other supports for people with MR and their families Definitions and Classifications of MR y There has been debate over the definition and classification of MR for several decades y Inconsistency in data collection with great differences in reported prevalence of MR may be partially attributable to the revisions and variations in some major definition and classification system Variations in Major Definition and Classification Systems y The traditional approach considers MR as a characteristic of a person and a condition with the source of difficulty lying essentially within the individual y Tends to define MR on the basis of either a medical model or a statistical modely The medical model focuses on pathology which defines MR by the presence of pathological symptoms y The statistical model defines MR by identifying a certain group of the population as abnormal using comparison of an individuals performance and the performance of a standardized norm group y Statistical model measures severity of MR by standardized tests y The latest ninth revision of the definition and classification of MR by AAMR has taken significant steps away from a clinically oriented perspective towards a multidimensional approach in defining MR y Although the new revision maintains the three key criterialow general intellectual functioning as measured by IQ score difficulties in adaptive behaviour and the conditions manifesting before age 18it puts more emphasis on functional and environmental considerations and less emphasis on an individuals deficiencyy Under the new AAMR definition the concept of adaptive behaviour is expanded with the specification of ten applicable skill areas relating to ageappropriate functioning of the individual in the community y The new AAMR definition replaced the previous classification of severity with a new concept of intensity of support requiredy The IQ cut off score was set up to 84 in the fifth revision and was reduced to approximately 70 in the sixth and eighth revisionsy The latest ninth revision defines significantly subaverage as IQ standard scores of approximately 70 to 75 or below y The APA has modified its recent fourth version by incorporating the ten adaptive skill areas of the new AAMR manual into its general definition of MR y The ICD10 manual also points out that intellectual abilities and rehabilitation may change over time and may improve by training and rehabilitation so diagnosis should be based on the current levels of functioning y The intellectual impairments concept in International classification of impairment disabilities and handicap ICIDH covers a wide range of impairments and syndromes involving impairments in intelligence memory and thinkingy MR is considered as one of the subcategories of intelligence impairments y The ICIDH definition of intellectual impairments excludes impairments of language and learning y In the revised version of the ICIDH MR is classified as part of intellectual functions together with intellectual growth intellectual retardation and dementia while memory thought and higher level of cognitive functions are includedy Neither of the who WHO manuals has a specified age as a cutoff point for the developmental period to define MR in contrast to the AAMR definition that requires the diagnosis of MR before age 18 Diversity in key criteria for case identification and classification y The debate over the key criteria to define cases has implications for epidemiological research because prevalence estimates may be influenced by changes in those criteria Zigler and colleagues felt that MR should be defined only on the basis of cognitive skills and that inclusion of social adaption y Barnett 1986 on the other hand agreed that the fundamental property of MR was cognitive inefficiency but felt that to operationalize this the cultural context and environment had to be taken into account y In practice adaptive behaviour is more likely to be ignored in epidemiological research of MR y Many studies use IQ score as the sole criterion to estimate prevalence of MR y This is partly because there are not totally objective and standardized measures of social adaptive behaviour particularly in different socioeconomic and cultural environments y Replacing the levels of IQ score with intensities of supports in severity measures in the new AAMR manual has been criticized in that it makes it more difficult to differentiate groups with mild from those with severe or profound MR y McLaren and Bryson claimed that this distinction is not clear cut and that there is likely to be an etiological role from the interaction of psychosocial and biological factors
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