In a study of the stability of IQ scores, a large group of people was tested at age 18 and the same people were tested again at age 35. At both age 18 and age 35, the average IQ score was around 100, with a standard deviation around 15. The scatterplot was elliptical, with a correlation coefficient around 0.80.
Calculate the residual standard deviation for predicting a person's IQ score at age 35 from his/her IQ score at age 18.