A solution to limitations of cognitive testing in children with intellectual disabilities: the case of fragile X syndrome
Open accessAbstract
Intelligence testing in children with intellectual disabilities (ID) has significant limitations. The normative samples of widely used intelligence tests, such as the Wechsler Intelligence Scales, rarely include an adequate number of subjects with ID needed to provide sensitive measurement in the very low ability range, and they are highly subject to floor effects. The IQ measurement problems in these children prevent characterization of strengths and weaknesses, poorer estimates of cognitive abilities in research applications, and in clinical settings, limited utility for assessment, prognosis estimation, and planning intervention. Here, we examined the sensitivity of the Wechsler Intelligence Scale for Children (WISC-III) in a large sample of children with fragile X syndrome (FXS), the most common cause of inherited ID. The WISC-III was administered to 217 children with FXS (age 6–17 years, 83 girls and 134 boys). Using raw norms data obtained with permission from the Psychological Corporation, we calculated normalized scores representing each participant’s actual deviation from the standardization sample using a z -score transformation. To validate this approach, we compared correlations between the new normalized scores versus the usual standard scores with a measure of adaptive behavior (Vineland Adaptive Behavior Scales) and with a genetic measure specific to FXS ( FMR1 protein or FMRP). The distribution of WISC-III standard scores showed significant skewing with floor effects in a high proportion of participants, especially males (64.9%–94.0% across subtests). With the z-score normalization, the flooring problems were eliminated and scores were normally distributed. Furthermore, we found correlations between cognitive performance and adaptive behavior, and between cognition and FMRP that were very much improved when using these normalized scores in contrast to the usual standardized scores. The results of this study show that meaningful variation in intellectual ability in children with FXS, and probably other populations of children with neurodevelopmental disorders, is obscured by the usual translation of raw scores into standardized scores. A method of raw score transformation may improve the characterization of cognitive functioning in ID populations, especially for research applications.
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Previous studies using the NIHTB-CB in ID have used a z-score deviation scoring approach.
“Alternatively, previous studies using the NIHTB-CB in ID have used a z-score deviation scoring approach (Hessl et al., 2009; Shields et al., 2020).”
From NIH Toolbox Cognition Battery Feasibility in Individuals With Williams Syndrome (2022) · cites this paper as Hessl et al., 2009
The Methods (Statistical methods; page unavailable) describe calculating age-based z-score deviations for children with intellectual disabilities. But the Methods (Measures; page unavailable) identify the test as the WISC-III, not the NIHTB-CB. This source supports the scoring approach, but not the claim that it was used with the NIHTB-CB. (LLM, conf=0.89)
Experimental — accuracy not yet measuredA particular challenge for measuring change in intellectual disability is the potential for floor effects, which may lead to a mischaracterization of a potentially meaningful change over time in individuals with intellectual disability.
“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 Hessl et al., 2009
Re-scoring of the Wechsler Intelligence Scale for Children and the Stanford-Binet for populations with intellectual disability has been completed and yields more sensitive estimates of true ability below the traditional test floor.
“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 [92]
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- Authors
- Hessl, D., Nguyen, D. V., Green, C., Chavez, A., Tassone, F., Hagerman, R. J., Senturk, D., Schneider, A., Lightbody, A., Reiss, A. L., & Hall, S.
- Affiliations
- University of California Davis Medical Center (US)
- University of California, Davis (US)
- Stanford Medicine (US)
- Corresponding author
- Hessl, David
- ORCID
- Venue
- Journal of Neurodevelopmental Disorders
- Published
- 2008-11-22
- Volume
- 1
- Issue
- 1
- Pages
- 33-45
- ISSN
- 1866-1947 (ISSN-L)
- 1866-1955
- PMC
- PMC2768415
- OpenAlex
- W1979516671
- Cited by
- 182
- References
- 27
- Citations by year
- 2026: 5 · 2025: 3 · 2024: 9 · 2023: 13 · 2022: 15 · 2021: 15 · 2020: 14 · 2019: 13 · 2018: 15 · 2017: 14 · 2016: 7 · 2015: 11 · 2014: 17 · 2013: 7 · 2012: 10
- FWCI
- 2.63
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Locations
- Journal of Neurodevelopmental Disorders (publishedVersion, cc-by)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- eScholarship (California Digital Library) (submittedVersion, cc-by)
- Funders
- National Institutes of Health (MH50047, HD02274, RR024992)
- National Institute on Aging (RL1AG032119, RL1AG032115)
- National Institute of Mental Health (R01MH064708, K23MH077554, R01MH050047)
- National Institute of Dental and Craniofacial Research (DE19583, UL1DE019583)
- National Center for Research Resources (HD02274, AG032115, AG032119, RR-024992, UL1RR024992)
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (P30HD002274, R01HD036071)
- Topics
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Genetics › Genetics and Neurodevelopmental Disorders
- Health Sciences › Medicine › Public Health, Environmental and Occupational Health › Down syndrome and intellectual disability research
- Social Sciences › Psychology › Experimental and Cognitive Psychology › Cognitive Abilities and Testing
- Keywords
- fragile X syndrome, intellectual disability, WISC-III, floor effect, Z-score normalization, Z-score, FMRP, adaptive behavior, cognitive assessment, children
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