An article in Lubrication Engineering (December 1990) describes the results of an experiment designed to investigate the effects of carbon material properties on the progression of blisters on carbon face seals. The carbon face seals are used extensively in equipment such as air turbine starters. Five different carbon materials were tested, and the surface roughness was measured. The data are as follows:

(a) Does carbon material type have an effect on mean surface roughness? Use a =0.05.
(b) Find the residuals for this experiment. Does a normal probability plot of the residuals indicate any problem with the normality assumption?
(c) Plot the residuals versus yij. Comment on the plot.
(d) Find a 95% confidence interval on the difference between the mean surface roughness between the EC10 and the EC1 carbon grades.

Carbon Material Surface Roughness Type 0.36 EC10 0.50 0.55 0.55 EC10A 0.31 0.07 0.25 0.18 0.56 0.20 0.12 EC4 0.20 0.28 ECI 0.10 0.16