title: Systematic review of prevalence studies of autism spectrum disorders.
abstract: OBJECTIVE
  To quantitatively examine the influence of study methodology and population characteristics on prevalence estimates of autism spectrum disorders.
  
  
  METHODS
  Electronic databases and bibliographies were searched and identified papers evaluated against inclusion criteria. Two groups of studies estimated the prevalence of typical autism and all autism spectrum disorders (ASD). The extent of variation among studies and overall prevalence were estimated using meta-analysis. The influence of methodological factors and population characteristics on estimated prevalence was investigated using meta-regression and summarised as odds ratios (OR).
  
  
  RESULTS
  Forty studies met inclusion criteria, of which 37 estimated the prevalence of typical autism, and 23 the prevalence of all ASD. A high degree of heterogeneity among studies was observed. The overall random effects estimate of prevalence across studies of typical autism was 7.1 per 10,000 (95% CI 1.6 to 30.6) and of all ASD was 20.0 per 10,000 (95% CI 4.9 to 82.1). Diagnostic criteria used (ICD-10 or DSM-IV versus other; OR = 3.36, 95% CI 2.07 to 5.46), age of the children screened (OR = 0.91 per year, 95% CI 0.83 to 0.99), and study location (e.g. Japan versus North America; OR = 3.60, 95% CI 1.73 to 7.46) were all significantly associated with prevalence of typical autism. Diagnostic criteria, age of the sample, and urban or rural location were associated with estimated prevalence of all ASD.
  
  
  CONCLUSIONS
  Sixty one per cent of the variation in prevalence estimates of typical autism was explained by these models. Diagnostic criteria used, age of children screened, and study location may be acting as proxies for other study characteristics and require further investigation.
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, UK. j.g.williams.97@cantab.net
authors.forename: J G
authors.initials: JG
authors.lastname: Williams
authors.forename: J P T
authors.initials: JP
authors.lastname: Higgins
authors.forename: C E G
authors.initials: CE
authors.lastname: Brayne
mesh_terms: D000367:Age Factors; D001321:Autistic Disorder / Q000175:diagnosis / Q000453:epidemiology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D015995:Prevalence; D012107:Research Design; D012423:Rural Health / Q000706:statistics & numerical data; D014504:Urban Health / Q000706:statistics & numerical data