13.28 ● Exercise 13.21 gave data on x 5 nerve firing frequency and y 5 pleasantness rating when nerves were stimulated by a light brushing stroke on the forearm. The x values and the corresponding residuals from a simple linear regression are as follows:

Firing Frequency, x Standardized Residual 23 21.83 24 0.04 22 1.45 25 0.20 27 21.07 28 1.19 34 20.24 33 20.13 36 20.81 34 1.17

a. Construct a standardized residual plot. Does the plot exhibit any unusual features?

b. A normal probability plot of the standardized residuals follows. Based on this plot, do you think it is reasonable to assume that the error distribution is approximately normal? Explain.