Consider the regression model fit to the gray range modulation data in Exercise 12-19. Use the useful range as the response.

(a) What proportion of total variability is explained by this model?

(b) Construct a normal probability plot of the residuals. What conclusion can you draw from this plot?

(c) Plot the residuals versus y and versus each regressor, and comment on model adequacy.

(d) Calculate Cook€™s distance for the observations in this data set. Are there any influential points in these data?

Brightness (%): Contrast (%): Useful range (ng): 96 50 50 112 54 61 65 100 100 100 50 57 54 50 65 80 96 80 155 25 35 26 56 80 70 144 255