title: Quantifying dimensions in autism: a factor-analytic study.
abstract: OBJECTIVE
  The objective of this study was to determine whether the phenotypic variation in autism and the related pervasive developmental disorders (PDDs) is a unitary construct or whether it is composed of distinct dimensions of autistic symptoms and measures of level of functioning.
  
  
  METHODS
  One hundred twenty-nine children with autism and other forms of PDD from two samples with different inclusion criteria were assessed with the Vineland Adaptive Behavior Scales to measure level of functioning and the Autism Diagnostic Interview to measure severity of autistic behaviors. A factor analysis with varimax rotation was performed on each sample, separately and combined.
  
  
  RESULTS
  Two factors emerged; one representing autistic symptoms and another representing level of functioning. The factor structure was remarkably similar and robust to variations in ascertainment and inclusion criteria between the samples. The validity of the distinction was supported by differences between males and females on the symptom factor, but not on the level of functioning factor. IQ was modestly correlated with level of functioning, but not with symptoms.
  
  
  CONCLUSIONS
  The phenotypic variation seen in autism/PDD is composed of at least two different dimensions of autistic symptoms and level of functioning. The implications of this dimensional heterogeneity for research, classification, and clinical practice are discussed.
authors.affiliation: Department of Psychiatry and Behavioral Neurosciences, McMaster University, Hamilton, Ontario, Canada.
authors.forename: Peter
authors.initials: P
authors.lastname: Szatmari
authors.forename: Chantal
authors.initials: C
authors.lastname: Mérette
authors.forename: Susan E
authors.initials: SE
authors.lastname: Bryson
authors.forename: Jacques
authors.initials: J
authors.lastname: Thivierge
authors.forename: Marc-Andre
authors.initials: MA
authors.lastname: Roy
authors.forename: Mireille
authors.initials: M
authors.lastname: Cayer
authors.forename: Michel
authors.initials: M
authors.lastname: Maziade
mesh_terms: D000293:Adolescent; D001321:Autistic Disorder / Q000175:diagnosis* / Q000523:psychology; D002648:Child; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D012720:Severity of Illness Index; D012951:Socialization; D011795:Surveys and Questionnaires