title: The autism epidemic: fact or artifact?
abstract: OBJECTIVE
  We provide an illustration of how changes in methodological factors may produce variations in the frequency of autistic disorder (AD) over time and project how much of the observed increase in the frequency of AD may be explained by methodological factors.
  
  
  METHODS
  Using a prediction analysis, we calculate how broadening diagnostic criteria, younger age at diagnosis, and improved efficiency of case ascertainment could produce temporal trends in the incidence and prevalence of AD, measured by calendar year and by year of birth, in a hypothetical population of children 0 to 18 across the years 1950 to 2020.
  
  
  RESULTS
  Time trend studies report an increase as large as 11.0-fold over a 13-year period for AD. Conservative changes in the three methodological factors produced increases in the frequency of AD ranging from 2.1- to 28.8-fold. Measures of frequency by year of birth show the largest magnitude of increase; predicted prevalence by calendar year and to age 4 by year of birth are influenced by changes in the distribution of age at diagnosis, but 1-year incidence and prevalence to age 12 are not.
  
  
  CONCLUSIONS
  Methodological factors may explain the observed increases in AD over time. To increase confidence in reports of time trends, we recommend particular frequency measures and study circumstances.
authors.affiliation: All of the authors are affiliated with the Department of Epidemiology, Mailman School of Public Health, Columbia University, New York; Dr. Wazana is also affiliated with the Department of Psychiatry at the Montreal Children's Hospital, Montreal, Quebec, Canada; Drs. Bresnahan and Kline are also affiliated with Epidemiology of Developmental Brain Disorders Department, New York State Psychiatric Institute, New York. Electronic address: ashley.wazana@mail.mcgill.ca.
authors.forename: Ashley
authors.initials: A
authors.lastname: Wazana
authors.affiliation: All of the authors are affiliated with the Department of Epidemiology, Mailman School of Public Health, Columbia University, New York; Dr. Wazana is also affiliated with the Department of Psychiatry at the Montreal Children's Hospital, Montreal, Quebec, Canada; Drs. Bresnahan and Kline are also affiliated with Epidemiology of Developmental Brain Disorders Department, New York State Psychiatric Institute, New York.
authors.forename: Michaeline
authors.initials: M
authors.lastname: Bresnahan
authors.affiliation: All of the authors are affiliated with the Department of Epidemiology, Mailman School of Public Health, Columbia University, New York; Dr. Wazana is also affiliated with the Department of Psychiatry at the Montreal Children's Hospital, Montreal, Quebec, Canada; Drs. Bresnahan and Kline are also affiliated with Epidemiology of Developmental Brain Disorders Department, New York State Psychiatric Institute, New York.
authors.forename: Jennie
authors.initials: J
authors.lastname: Kline
mesh_terms: D000293:Adolescent; D001321:Autistic Disorder / Q000175:diagnosis / Q000453:epidemiology*; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D039721:Diagnostic and Statistical Manual of Mental Disorders; D015981:Epidemiologic Factors; D014943:Global Health; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008962:Models, Theoretical; D012107:Research Design; D013997:Time Factors