title: Using developmental trajectories to understand developmental disorders.
abstract: OBJECTIVE
  In this article, the authors present a tutorial on the use of developmental trajectories for studying language and cognitive impairments in developmental disorders and compare this method with the use of matching.
  
  
  METHODS
  The authors assess the strengths, limitations, and practical implications of each method. The contrast between the methodologies is highlighted using the example of developmental delay and the criteria used to distinguish delay from atypical development.
  
  
  RESULTS
  The authors argue for the utility of the trajectory approach, using illustrations from studies investigating language and cognitive impairments in individuals with Williams syndrome, Down syndrome, and autism spectrum disorder.
  
  
  CONCLUSIONS
  Two conclusions were reached: (a) An understanding of the underlying mechanism will be furthered by the richer descriptive vocabulary provided by the trajectories approach (e.g., in distinguishing different types of delay) and (b) an optimal design for studying developmental disorders is to combine initial cross-sectional designs with longitudinal follow-up.
authors.affiliation: Developmental Neurocognition Laboratoy, School of Psychology, Birkbeck College, University of London, London, UK. m.thomas@bbk.ac.uk
authors.forename: Michael S C
authors.initials: MS
authors.lastname: Thomas
authors.forename: Dagmara
authors.initials: D
authors.lastname: Annaz
authors.forename: Daniel
authors.initials: D
authors.lastname: Ansari
authors.forename: Gaia
authors.initials: G
authors.lastname: Scerif
authors.forename: Chris
authors.initials: C
authors.lastname: Jarrold
authors.forename: Annette
authors.initials: A
authors.lastname: Karmiloff-Smith
mesh_terms: D002648:Child; D002657:Child Development; D002675:Child, Preschool; D003072:Cognition Disorders / Q000175:diagnosis / Q000523:psychology; D003430:Cross-Sectional Studies; D002658:Developmental Disabilities / Q000175:diagnosis* / Q000523:psychology*; D004314:Down Syndrome / Q000523:psychology; D006801:Humans; D007806:Language Disorders / Q000175:diagnosis / Q000523:psychology; D008137:Longitudinal Studies; D008569:Memory Disorders / Q000523:psychology; D018980:Williams Syndrome / Q000523:psychology