Monday, March 17, 2014

Hall vs. Florida: IQ, the death penalty, and margin of error (edited 5/27/14)

Here is Think Progress' story about a U.S. Supreme Court case that hinges on statistics. The case centers around death row inmate Freddy Lee Hall. He was sentenced to death in Florida for the murder of Karol Hurst in 1978. This isn't in dispute. What is in dispute is whether or not Hall qualifies as mentally retarded and, thus, should be exempt from the death penalty per Virginia vs. Atkins.

So, this is an example relevant to any number of psychology classes (developmental, ethics, psychology and the law, etc.). It is relevant to a statistics class because the main thrust of the argument has to do with the margin of error associated with the IQ test that designated Hall as having an IQ of 71. In order to qualify as mentally retarded in Florida, an individual has to have an IQ of 70 or lower. So, at first blush, Hall is out of luck. Until his lawyers bring up the fact that the margin of error on this test is +/- 5 points. This is a good example of confidence intervals/margin of error in a very high stakes situation. 

If you want to hear the very statsy oral argument, listen to it here. It is interesting to hear a fairly simple statistical construct (standard of error) described as by a lawyer. Frankly, it is unsettling to listen to the lawyer try to explain statistics. He conflates standard error and standard deviation, doesn't understand what 95% confidence means.

Edited 5/27 to add:

"Held: The State's threshold requirement, as interpreted by the Florida Supreme Court, is unconstitutional. Pp. 5-22."  (meaning, the current IQ standards at which a person would be executed is considered unconstitutional)

Read the rest of the Supreme Court's decision here. It prominently features the problem of standard error of measurement (margin of error) and the limitations of IQ testing in the decision.