Dyck, MJ and Piek, JP and Patrick, J, The validity of psychiatric diagnoses: The case of 'specific' developmental disorders, Research in Developmental Disabilities, 32, (6) pp. 2704-2713. ISSN 0891-4222 (2011) [Refereed Article]
Copyright 2011 Elsevier.
Official URL: http://dx.doi.org/10.1016/j.ridd.2011.06.001
We tested whether developmental coordination disorder (DCD) and mixed receptive
expressive language disorder (RELD) are valid diagnoses by assessing whether they are
separated from each other, from other childhood disorders, and from normality by natural
boundaries termed zones of rarity. Standardized measures of intelligence, language, motor
skills, social cognition, and executive functioning were administered to children with DCD
(n = 22), RELD (n = 30), autistic disorder (n = 30), mental retardation (n = 24), attention
deficit/hyperactivity disorder (n = 53) and to a representative sample of children (n = 449).
Discriminant function scores were used to test whether there were zones of rarity between
the DCD, RELD, and other groups. DCD and RELD were reliably distinguishable only from
the mental retardation group. Cluster and latent class analyses both resulted in only two
clusters or classes being identified, one consisting mainly of typical children and the other
of children with a disorder. Fifty percent of children in the DCD group and 20% in the RELD
group were clustered with typical children. There was no evidence of zones of rarity
between disorders. Rather, with the exception of mental retardation, the results imply
there are no natural boundaries between disorders or between disorders and normality.
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