This post at Science Evidence discusses a recent death penalty case in California (People v. Superior Court (Vidal), 40 Cal. 4th 999, 155 P. 3d 259, 56 Cal. Rptr. 3d 851 (2007)). Briefly, defendant Vidal was charged with capital murder and he moved for a hearing regarding his alleged mental retardation. Vidal had received numerous IQ tests over the years producing various IQ scores. Vidal's experts claimed that Vidal's full-scale IQ score which did not fall below the gold-standard of 70 was an inaccurate indication of his true intelligence and argued that Vidal's Verbal IQ was a better gauge of Vidal's intelligence. The trial court accepted this view holding Vidal mentally retarded. Upon appeal the Court of Appeal reversed, holding the trial court's conclusion erroneous. Vidal appealed and the California Supreme Court reversed, holding:

That Vidal’s Full Scale Intelligence Quotient on Wechsler IQ tests (Full Scale IQ) has generally been above the range considered to show mental retardation does not, as a matter of law, dictate a finding he is not mentally retarded.

While the post's author finds the Court of Appeal opinion troubling on procedural grounds, from a scientific standpoint, the Court of Appeal got it right. Psychological tests are developed to be used strictly in accord with their formal scoring procedures. Any deviations risk the test becoming invalid and unreliable. It is true that verbal intelligence is considered by some to be a sufficient measure of intelligence for those with significant mental retardation. But the IQ test in question, the WAIS, is not designed to be used in this manner. Moreover, verbal measures of intelligence are considered by most experts to be less valid and reliable than measures that include performance tasks. To put it bluntly, the WAIS, which measures verbal and performance, is considered the gold standard in intellectual quotient assessment whereas exclusive verbal measures are frequently used not because of their superior psychometric properties but because they are fast and easy to use.