The prevalence of autism spectrum disorders: impact of diagnostic instrument and non-response bias.
BACKGROUND
A large part of the variability in rates of autism spectrum disorders (ASD) across studies is non-aetiologic, and can be explained by differences in diagnostic criteria, case-finding method, and other issues of study design.


OBJECTIVE
To investigate the effects on ASD prevalence of two methodological issues; non-response bias and case ascertainment. We compared the findings of using a semi-structured parent interview versus in-depth clinical assessment, including an ASD specific interview. We further explored whether including information on non-responders affected the ASD prevalence estimate.


METHODS
A total population of 7- to 9-year olds (N = 9,430) was screened for ASD with the autism spectrum screening questionnaire (ASSQ) in the Bergen Child Study (BCS). Children scoring above the 98th percentile on parent and/or teacher ASSQ were invited to participate in the second and subsequently in the third phase of the BCS where they were assessed for ASD using the Development and Well-Being Assessment (DAWBA), and the Diagnostic Interview for Social and Communication disorders (DISCO), respectively.


RESULTS
Clinical assessment using DISCO confirmed all DAWBA ASD cases, but also diagnosed additional cases. DISCO-generated minimum prevalence for ASD was 0.21%, whereas estimated prevalence was 0.72%, increasing to 0.87% when adjusting for non-responders. The DAWBA estimate for the same population was 0.44%.


CONCLUSIONS
Large variances in prevalence rates across studies can be explained by methodological differences. Both information about assessment method and non-response are crucial when interpreting prevalence rates of ASD.
Posserud
M
M
Centre for Child and Adolescent Mental Health, Unifob Health, Jonas Lies vei 91, 5009, Bergen, Norway. maj-britt.posserud@rbup.uib.no
Lundervold
Astri J
AJ
Lie
Stein Atle
SA
Gillberg
Christopher
C
D020817:Asperger Syndrome / Q000175:diagnosis / Q000453:epidemiology / Q000523:psychology; D001321:Autistic Disorder / Q000175:diagnosis / Q000453:epidemiology / Q000523:psychology; D015982:Bias; D002648:Child; D002659:Child Development Disorders, Pervasive / Q000175:diagnosis* / Q000453:epidemiology / Q000523:psychology; D003625:Data Collection / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008403:Mass Screening / Q000706:statistics & numerical data; D009483:Neuropsychological Tests; D015995:Prevalence; D011569:Psychiatric Status Rating Scales / Q000706:statistics & numerical data; D011594:Psychometrics; D015203:Reproducibility of Results; D012107:Research Design; D011795:Surveys and Questionnaires; D013663:Teaching / Q000706:statistics & numerical data