Person Ability Scores as an Alternative to Norm-Referenced Scores as Outcome Measures in Studies of Neurodevelopmental Disorders.
Trade norm-referenced scores for IRT ability scores to see real growth in clients with developmental disabilities.
01Research in Context
What this study did
Mulder et al. (2020) wrote a theory paper. They asked: why do norm-referenced scores hide real progress in kids with autism or delays?
They explain person ability scores from IRT. These scores track each child’s own growth curve instead of ranking them against same-age peers.
What they found
The team argues that norm scores can look flat even when a child is learning new skills. Ability scores pick up smaller, steady gains.
In short, switch lenses and you see real change.
How this fits with other research
Frazier et al. (2025) took the idea and ran with it. They built reliable-change indices for the NET tool, giving you a ready-made way to use sensitive metrics in practice.
Green et al. (2020) show the problem A et al. warn about: the DAS-II spatial subtest gives autistic kids inflated scores. Using ability scores instead could dodge that bias.
Berry-Kravis et al. (2008) seem to push the old way, praising standard cognitive tests for good test-retest reliability. The papers don’t clash; Elizabeth’s group looked at stability while A et al. look at growth detection—different questions.
Why it matters
Next time you probe for change, ask your software for IRT ability scores, not standard scores. You will spot gains sooner and adjust teaching plans faster.
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02At a glance
03Original abstract
Although norm-referenced scores are essential to the identification of disability, they possess several features which affect their sensitivity to change. Norm-referenced scores often decrease over time among people with neurodevelopmental disorders who exhibit slower-than-average increases in ability. Further, the reliability of norm-referenced scores is lower at the tails of the distribution, resulting in floor effects and increased measurement error for people with neurodevelopmental disorders. In contrast, the person ability scores generated during the process of constructing a standardized test with item response theory are designed to assess change. We illustrate these limitations of norm-referenced scores, and relative advantages of ability scores, using data from studies of autism spectrum disorder and creatine transporter deficiency.
American journal on intellectual and developmental disabilities, 2020 · doi:10.1136/jmedgenet-2013-101658