Beyond the floor effect on the Wechsler Intelligence Scale for Children--4th Ed. (WISC-IV): calculating IQ and Indexes of subjects presenting a floored pattern of results.
You can now pull real, differentiated IQ and Index scores from the WISC-IV even when a child floors every subtest.
01Research in Context
What this study did
Boudreau et al. (2015) built a new way to score the WISC-IV for kids with severe intellectual disability.
Instead of giving every floored subtest a scaled score of 1, they extrapolate from raw items.
The paper shows the math and checks that the new numbers still measure what they should.
What they found
The method turns a pile of "1" scores into real IQ and Index scores you can tell apart.
Kids who looked equal before now show separate verbal, non-verbal, and processing-speed profiles.
How this fits with other research
Barton et al. (2019) tried a different fix: use age-equivalent or Z scores. Both teams kill the floor effect, so you now have two tools, not one.
Whitaker (2008) warns that even good low scores can drift over time—about 1 in 7 kids move 10+ points—so reassess even the new scores every few years.
Umphress (2008) shows the RIAS can give higher IQs than the Wechsler tests in adults; A’s method keeps you on the WISC-IV while still getting usable numbers.
Why it matters
You no longer have to write "<70" and guess what the child really knows. Plug the raw items into the formula and you get separate Indexes for planning goals and tracking progress. It takes five extra minutes and gives you a clearer picture for the next IEP meeting.
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02At a glance
03Original abstract
BACKGROUND: It is now widely known that children with severe intellectual disability show a 'floor effect' on the Wechsler scales. This effect emerges because the practice of transforming raw scores into scaled scores eliminates any variability present in participants with low intellectual ability and because intelligence quotient (IQ) scores are limited insofar as they do not measure scores lower than 40. METHOD: Following Hessl et al.'s results, the present authors propose a method for the computation of the Wechsler Intelligence Scale for Children--4th Ed. (WISC-IV)'s IQ and Indexes in intellectually disabled participants affected by a floored pattern of results. The Italian standardization sample (n = 2200) for the WISC-IV was used. The method presented in this study highlights the limits of the 'floor effect' of the WISC-IV in children with serious intellectual disability who present a profile with weighted scores of 1 in all the subtests despite some variability in the raw scores. RESULTS: Such method eliminates the floor effect of the scale and therefore makes it possible to analyse the strengths and weaknesses of the WISC-IV's Indexes in these participants. CONCLUSIONS: The Authors reflect on clinical utility of this method and on the meaning of raw score of 0 on subtest.
Journal of intellectual disability research : JIDR, 2015 · doi:10.1111/jir.12150