Comparison of IQ measures among children and adults with down syndrome

There is a paucity of research comparing estimated Intelligence Quotient (IQ) or intellectual disability (ID) level based on different tests in individuals with Down syndrome (DS). Yet, these differences have important implications because levels of intellectual function are frequently used as inclusion or exclusion criteria for research and clinical trials, and often used to control for differences in cognitive abilities across individuals with DS. This study compared floor effects, ID classifications, and hypothetical inclusion/exclusion cut-offs of standard scores and age equivalents (AEs) for two commonly used IQ tests. Participants were 276 individuals with DS ranging in age from 6 to 61 years (M age = 22.00 years, SD = 13.19) drawn from two large multisite studies. Individuals with DS completed both the Kaufman Brief Intelligence Test, Second Edition (KBIT-2) and the Stanford-Binet Intelligence Scales, Fifth Edition (SB-5). Floor effects were problematic with traditional standard scores on both KBIT-2 and SB-5; however, floor effects were reduced when using developmental AEs and the SB-5 deviation scoring. ID classifications varied based on IQ test, with participants more likely to be classified as having moderate ID than severe/profound ID based on their KBIT-2 score compared to the SB-5. When considering who would be theoretically included or excluded based on specific study criteria, the KBIT-2 was more inclusive compared to the SB-5 for both standard/deviation scores and AE cutoffs. Overall IQ composite scores on the KBIT-2 and SB-5 were similar, yet there were instances where scores did not align ( n = 33; 12%). There were considerable floor effects on both IQ measure standard scores and ID classifications varied based on the test used to categorize mild, moderate, and severe/profound ID. KBIT-2 Nonverbal standard scores and Verbal AEs were the least restrictive in examination of hypothetical study eligibility cutoffs. Findings can inform researchers on strengths and limitations of specific IQ measurement tools, understand variability in floor effects or ID classifications based on a certain test, and guide selection of IQ measures based on the specific needs of future studies or clinical trials.