Yuri computes the mean and standard deviation for the sample data set {12, 14, 9, 21}. He finds the mean is 14. His steps for finding the standard deviation are below.
[tex]\[ s = \sqrt{\frac{(12-14)^2 + (14-14)^2 + (9-14)^2 + (21-14)^2}{4}} \][/tex]
[tex]\[ = \sqrt{\frac{(-2)^2 + (0)^2 + (-5)^2 + (7)^2}{4}} \][/tex]
[tex]\[ = \sqrt{\frac{4 + 0 + 25 + 49}{4}} \][/tex]
[tex]\[ = \sqrt{\frac{78}{4}} \][/tex]
[tex]\[ = \sqrt{19.5} \][/tex]
What is the first error he made in computing the standard deviation?