Measurement invariance in the assessment of people with an intellectual disability.
The WAIS-III’s four-factor IQ model breaks down in adults with intellectual disability, so treat their scores as approximate.
01Research in Context
What this study did
MacLean et al. (2011) ran a confirmatory factor analysis on the WAIS-III in adults with intellectual disability. They wanted to see if the test’s usual four-factor structure still fit this group.
The WAIS-III is the most common IQ test in clinics. If the factors shift, the scores may not mean the same thing.
What they found
The four-factor model showed poor fit. Factor loadings and item patterns were different from the general population.
Bottom line: the WAIS-III does not measure IQ the same way in adults with ID, so standard scores may be invalid.
How this fits with other research
Maïano et al. (2011) tested the short Physical Self-Inventory in 248 teens and young adults with ID. They found strong fit and full measurement invariance. Same method, same group, opposite result.
The difference is the tool. The PSI-VS-ID was built for ID; the WAIS-III was not. This is an apparent contradiction that warns us to pick tests designed for the population.
Haynes et al. (2013) also reworked factor structure. They trimmed the five-factor SDQ down to three factors for kids with ID and got good fit. Together these papers show that shorter, simpler tools often work better.
Why it matters
If you give the WAIS-III to adults with ID, treat the full-scale IQ as a rough estimate, not a hard number. Use it to guide support level, not to gate services. When you need precise self-concept or emotional data, switch to tools like the PSI-VS-ID or the three-factor SDQ that have proven invariance in this group.
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02At a glance
03Original abstract
Intellectual assessment is central to the process of diagnosing an intellectual disability and the assessment process needs to be valid and reliable. One fundamental aspect of validity is that of measurement invariance, i.e. that the assessment measures the same thing in different populations. There are reasons to believe that measurement invariance of the Wechsler scales may not hold for people with an intellectual disability. Many of the issues which may influence factorial invariance are common to all versions of the scales. The present study, therefore, explored the factorial validity of the WAIS-III as used with people with an intellectual disability. Confirmatory factor analysis was used to assess goodness of fit of the proposed four factor model using 13 and 11 subtests. None of the indices used suggested a good fit for the model, indicating a lack of factorial validity and suggesting a lack of measurement invariance of the assessment with people with an intellectual disability. Several explanations for this and implications for other intellectual assessments were discussed.
Research in developmental disabilities, 2011 · doi:10.1016/j.ridd.2011.01.022