12.73 The dandruff study, continued. Refer to the previous exercise.

  1. (a) Plot the residuals versus case number (the first variable in the data set). Describe the plot. Is there any pattern that would cause you to question the assumption that the data are independent?

  2. (b) Examine the standard deviations for the four treatment groups. Is there a problem with the assumption of equal standard deviations for ANOVA in this data set? Explain your answer.

  3. (c) Create Normal quantile plots for each treatment group. What do you conclude from these plots?

  4. (d) Obtain the residuals from the analysis of variance and create a Normal quantile plot of these. What do you conclude?