When the U.S. Supreme Court decided in Atkins v. Virginia in 2002 that people who are mentally retarded (now called intellectually disabled) can’t be executed no matter how heinous the crime, it opened a can or worms regarding deciding who actually qualifies for that category. The line between that condition and the next level up (borderline intellectual functioning) is a matter of convention, not really science, so there is a range of disagreement.
A case now being briefed before the court, Hamm v. Smith, deals with the question of how to assess the IQ of someone who has been tested multiple times. The court briefly touched on that issue in 2014 in Hall v. Florida. The year before, Joel Schneider of Temple University proposed a method in a chapter of an edited book. The opinion of the court cited that chapter but brushed it off with the comment that his method is “a complicated endeavor.” Really? It’s not all that complicated. I ran the numbers myself on the data in the Smith case. It wasn’t simple, but it was simpler than computing my 2024 income tax return.
As a preliminary matter, the makers of IQ tests regularly publish a “standard error of measurement” (SEM). That number represents, in a statistical way, the scatter one could expect in giving a test multiple times to the same person or to multiple people with identical true IQs. It doesn’t account for a host of other possible errors such as incorrect administration of the test, poor testing conditions, transient mental or physical problems of an examinee having a bad day, or–the big one in criminal cases–malingering.
So, putting those aside, here is how we do the math on the Smith case with the Schneider method. Continue reading . . .