The prevalence of autism spectrum disorders: impact of diagnostic instrument and non-response bias
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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.
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.
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.
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%.
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.
KeywordsAutism spectrum disorders Prevalence Non-response bias Diagnostic instrument Assessment method
The present study was supported by fellowship from the Centre of Child and Adolescent Mental Health, Unifob Health, Bergen, and was also funded by the University of Bergen, the Norwegian Directorate for Health and Social Affairs, the Norwegian Research Council, and the Western Norway Regional Health Authority. We are grateful to the children, parents and teachers for participating in the BCS, and to the other members of the project group for making the study possible. We thank Jim Stevenson and Berit Hilt for helpful comments on the manuscript.
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