Quarreling and Quibbling over Psychometrics in Hall v. Florida (part 1)

In Hall v. Florida, 134 S.Ct. 1986 (2014), the Supreme Court struck down Florida’s practice of using an IQ score of 70 points or less as a dispositive measure of the level of the “intellectual disability” that precludes capital punishment. Justice Kennedy’s opinion (joined by Justices Breyer, Ginsburg, Sotomayor, and Kagan) summarizes the case pithily:

This Court has held that the Eighth and Fourteenth Amendments to the Constitution forbid the execution of persons with intellectual disability. Atkins v. Virginia, 536 U.S. 304, 321 (2002). Florida law defines intellectual disability to require an IQ test score of 70 or less. If, from test scores, a prisoner is deemed to have an IQ above 70, all further exploration of intellectual disability is foreclosed. This rigid rule, the Court now holds, creates an unacceptable risk that persons with intellectual disability will be executed, and thus is unconstitutional.

Id. at 1990. Led by Justice Alito, Chief Justice Roberts and Justices Scalia, Kennedy, and Thomas dissented. The Court, they maintained, was overruling Atkins and adopting “a uniform national rule that is both conceptually unsound and likely to result in confusion.” Id. at 2002 (Alito, J., dissenting). Among other things, the dissent warns that the Court “misunderstands” the statistical concepts of standard error and confidence intervals, id. at 2009, and that it therefore “makes factual mistakes that will surely confuse States attempting to comply with its opinion.” Id. at 2010.

The dissent has a point. Parts of the majority opinion are elliptical and potentially confusing. Nonetheless, some of the harsh critique is overdrawn. Moreover, Justice Alito's presentation of psychometric concepts also is hardly error-free, inviting a rejoinder of "tu quoque."

Therefore, in a series of postings yet to come, I will, in Justice Alito's words, "wade[] into technical matters that must be understood in order to see where the Court goes wrong." But I'll do the same for the dissenting opinion's presentation of these matters.

POSTINGS ON IQ SCORES AND CAPITAL PUNISHMENT

• Quarreling and Quibbling over Psychometrics in Hall v. Florida (part 1), May 29, 2014 (introduction)
• Quarreling and Quibbling over Psychometrics in Hall v. Florida (part 2), June 2, 2014 (on standard deviation)
• Quarreling and Quibbling over Psychometrics in Hall v. Florida (part 3), June 4, 2014 (on validity and the stability of the APA's diagnostic criteria)
• After Moore v. Texas Is a Single IQ Score Really Determinative?, March 29, 2017
• Muddling Through the Measurement of IQ, November 23, 2018