Different Standardized Instruments Produce Different IQ Results: Significant Policy Implications

An interesting and controversial article entitled “Stanford-Binet and WAIS IQ differences and their implications for adults with intellectual disability (aka mental retardation)” by Wayne Silverman et al. was published in Intelligence January 2010 (38, 242-248).  The essence of this research article indicates that the WAIS full scale IQ was higher than the Stanford-Binet Composite IQ with a mean difference of 16.7 points.

 The abstract of this article states: “Stanford-Binet and Wechsler Adult Intelligence Scale (WAIS) IQs were compared for a group of 74 adults with intellectual disability (ID).  In every case, WAIS Full Scale IQ was higher than the Stanford-Binet Composite IQ with a mean difference of 16.7 points.  These differences did not appear to be due to the lower minimum possible score for the Stanford-Binet.  Additional comparisons with other measures suggested that the WAIS might systematically underestimate severity of intellectual impairment.  Implications of these findings are discussed regarding determination of disability status, estimating prevalence of ID, assessing dementia and aging-related cognitive declines, and diagnosis of ID in forensic cases involving a possible death penalty.”

 The article identified four significant public policy implications regarding the discrepancy of the Stanford-Binet and the WAIS IQ testing.  These are summarized briefly here. 

 Disability determinations:  Eligibility for adults with ID for support through the Social Security Administration is demonstrated by: (a) a valid contemporaneously established IQ of 59 or less, (b) a valid IQ between 60 and 70 and another substantial physical or mental impairment, or (c) a valid IQ between 60 and 70 and substantial functional impairments.  Based upon the Stanford-Binet results, 94.6% of the sample in this study would be eligible for benefits on the basis of IQ testing alone (assuming that requirements of age of onset and economic need are also met).  However, using the WAIS only 60.8% of the sample would have qualified without additional evidence of disability.  The authors note: “Results of these two tests are clearly not equivalent for this population, and consensus is needed regarding a preferred ‘gold standard’ for SSA disability determination.”

 Prevalence of ID:  Adults are more often assessed with the WAIS, while school age children more often are assessed using either the Stanford-Binet or WISC.  If the scores for adult assessments tend to be, on average 7.5 points higher, simply because of the predominant use of the WAIS, the overall percentage of adults scoring 70 or below would change from 2.3% to 0.6%.  The authors note:  “[T]he public health significance of ID cannot be addressed effectively unless and until we determine the true size of the affected population and the nature of the associated impairments.”

 Declines with aging:  The authors note “Pre-morbid IQ should influence both selection of appropriate instruments for assessment of performance as well as judgments formed from clinical evaluations, and it is abundantly clear that the sets of Stanford-Binet and WAIS IQs described herein cannot both provide valid indications of performance expectations.”  There is a need to determine how best to use IQ test results to inform diagnoses.

 Death penalty cases:  The Supreme Court ruled, in Atkins v. Virginia (2002), that people with ID convicted of capital crimes are no longer subject to the death penalty.  Evidence of a “true” IQ of 70 or below is required as evidence of ID.  This research indicates that a Stanford-Binet is far more likely to support a diagnosis of ID.  While IQ tests should never be selected based upon expectations of a higher or lower result, psychologists are also charged with knowing which test provides the most valid estimate of true intelligence.

 Research conducted by Dr. Wayne Silverman et al. has indicated that there is a substantial difference in the results of IQ testing between the Stanford-Binet and the WAIS, with the WAIS indicating higher scores than the Stanford-Binet for individuals who have ID.  This research indicates that there can be major public policy implications because IQ testing is used for a number of reasons including disability determination, prevalence of ID, decline with aging, and death penalty cases.  Additionally, there are other public policy indications, such as eligibility criteria for service delivery and school accommodations. 

 More research needs to be conducted regarding IQ testing with persons who have ID.  We need further data to determine whether the Stanford-Binet and WAIS produce similar or different results.  Furthermore, other IQ scales also need to be compared with the Stanford-Binet and WAIS for persons who have ID.  This is an important issue that needs to have careful review and study as the public policy implications are huge

 Robert J. Fletcher, Founder & CEO, NADD

The NIH Public Access Author Manuscript of the Silverman et al. article is available at

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2854585/?tool=pubmed.