Selection bias on intellectual ability in autism research: a cross-sectional review and meta-analysis

Mol Autism. 2019 Mar 1;10:9. doi: 10.1186/s13229-019-0260-x. eCollection 2019.

Abstract

Background: Current global estimates suggest the proportion of the population with autism spectrum disorder (ASD) who have intellectual disability (ID) is approximately 50%. Our objective was to ascertain the existence of selection bias due to under-inclusion of populations with ID across all fields of autism research. A sub-goal was to evaluate inconsistencies in reporting of findings.

Methods: This review covers all original research published in 2016 in autism-specific journals with an impact factor greater than 3. Across 301 included studies, 100,245 participants had ASD. A random effects meta-analysis was used to estimate the proportion of participants without ID. Selection bias was defined as where more than 75% of participants did not have ID.

Results: Meta-analysis estimated 94% of all participants identified as being on the autism spectrum in the studies reviewed did not have ID (95% CI 0.91-0.97). Eight out of ten studies demonstrated selection bias against participants with ID. The reporting of participant characteristics was generally poor: information about participants' intellectual ability was absent in 38% of studies (n = 114). Where there was selection bias on ID, only 31% of studies mentioned lack of generalisability as a limitation.

Conclusions: We found selection bias against ID throughout all fields of autism research. We recommend transparent reporting about ID and strategies for inclusion for this much marginalised group.

Keywords: Autism; Autism spectrum disorder; Intellectual disability; Nosology; Selection bias.

Publication types

  • Meta-Analysis
  • Research Support, Non-U.S. Gov't
  • Systematic Review

MeSH terms

  • Autism Spectrum Disorder / complications*
  • Autism Spectrum Disorder / epidemiology
  • Biomedical Research / standards
  • Humans
  • Intellectual Disability / diagnosis*
  • Intellectual Disability / epidemiology
  • Patient Selection*
  • Selection Bias