Some people facing execution have had their IQ estimated multiple times. If an ability is measured more than once, there is an established way to combine all of the available information. These procedures are used in every psychological test ever published.
Some people who should not be eligible for execution are deemed eligible because psychologists are not combining IQ scores properly. The current standard of averaging the scores (or taking the median) is wrong. The correct procedure is the one we use in every other domain of psychological measurement, including in the computation of a single IQ score. When a person has been tested multiple times, this same procedure should be used to estimate a person’s IQ.
The correct calculation procedures are also explained on pages 289–291 of this chapter:
Schneider, W. J. (2013). Principles of assessment of aptitude and achievement. In D. Saklofske, C. Reynolds, & V. Schwean (Eds.), Oxford handbook of psychological assessment of children and adolescents (pp. 286–330). New York: Oxford University Press.