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Improving IQ measurement in intellectual disabilities using true deviation from population norms
Open accessAbstract
BACKGROUND: Intellectual disability (ID) is characterized by global cognitive deficits, yet the very IQ tests used to assess ID have limited range and precision in this population, especially for more impaired individuals. METHODS: We describe the development and validation of a method of raw z-score transformation (based on general population norms) that ameliorates floor effects and improves the precision of IQ measurement in ID using the Stanford Binet 5 (SB5) in fragile X syndrome (FXS; n = 106), the leading inherited cause of ID, and in individuals with idiopathic autism spectrum disorder (ASD; n = 205). We compared the distributional characteristics and Q-Q plots from the standardized scores with the deviation z-scores. Additionally, we examined the relationship between both scoring methods and multiple criterion measures. RESULTS: We found evidence that substantial and meaningful variation in cognitive ability on standardized IQ tests among individuals with ID is lost when converting raw scores to standardized scaled, index and IQ scores. Use of the deviation z- score method rectifies this problem, and accounts for significant additional variance in criterion validation measures, above and beyond the usual IQ scores. Additionally, individual and group-level cognitive strengths and weaknesses are recovered using deviation scores. CONCLUSION: Traditional methods for generating IQ scores in lower functioning individuals with ID are inaccurate and inadequate, leading to erroneously flat profiles. However assessment of cognitive abilities is substantially improved by measuring true deviation in performance from standardization sample norms. This work has important implications for standardized test development, clinical assessment, and research for which IQ is an important measure of interest in individuals with neurodevelopmental disorders and other forms of cognitive impairment.
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“However, a particular challenge for measuring change in ID is the potential for floor effects (Hessl et al., 2009; Sansone et al., 2014), which may lead to a mischaracterization of a potentially meaningful change over time in individuals with ID (Bishop et al., 2015; Thurm et al., 2020).”
From NIH Toolbox Cognition Battery Feasibility in Individuals With Williams Syndrome (2022) · cites this paper as Sansone et al., 2014
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“re-scoring of the Wechsler Intelligence Scale for Children (WISC)92 and Stanford–Binet10 for populations with intellectual disability has been completed, yielding much more sensitive estimates of true ability below the traditional test floor.”
From Drug development for neurodevelopmental disorders: lessons learned from fragile X syndrome (2018) · cites this paper as [10]
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Metadata
- Authors
- Sansone, S. M., Schneider, A., Bickel, E., Berry-Kravis, E., Prescott, C., & Hessl, D.
- Affiliations
- University of California Davis Medical Center (US)
- Rush University Medical Center (US)
- Corresponding author
- Hessl, David
- Venue
- Journal of Neurodevelopmental Disorders
- Published
- 2014-07-08
- Volume
- 6
- Issue
- 1
- Pages
- 16-16
- ISSN
- 1866-1947 (ISSN-L)
- 1866-1955
- PMC
- PMC4613563
- OpenAlex
- W2122684341
- Cited by
- 148
- References
- 34
- Citations by year
- 2026: 5 · 2025: 15 · 2024: 14 · 2023: 19 · 2022: 26 · 2021: 24 · 2020: 17 · 2019: 13 · 2018: 3 · 2017: 6 · 2016: 3 · 2015: 3
- FWCI
- 3.98
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- cc-by
- Locations
- Journal of Neurodevelopmental Disorders (publishedVersion, cc-by)
- Europe PMC (PubMed Central) (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- eScholarship (California Digital Library) (submittedVersion)
- eScholarship (California Digital Library) (submittedVersion, cc-by)
- Funders
- Autism Speaks
- National Fragile X Foundation
- University of California, Davis
- National Institute of Mental Health (U24MH081810)
- University of California Davis School of Medicine
- Topics
- Health Sciences › Medicine › Public Health, Environmental and Occupational Health › Down syndrome and intellectual disability research
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Genetics › Genetics and Neurodevelopmental Disorders
- Social Sciences › Psychology › Experimental and Cognitive Psychology › Cognitive Abilities and Testing
- Keywords
- intellectual disability, IQ assessment, floor effect, fragile X syndrome, autism spectrum disorder, neurodevelopmental disorders
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