64. Fretting is a wear process that results from tangential oscillatory movements of small amplitude in machine parts. The article “Grease Effect on Fretting Wear of Mild Steel”

(Industrial Lubrication and Tribology, 2008: 67–78)

included the following data on volume wear for base oils having four different viscosities.

Viscosity Wear 20.4 58.8 30.8 27.3 29.9 17.7 76.5 30.2 44.5 47.1 48.7 41.6 32.8 18.3 89.4 73.3 57.1 66.0 93.8 133.2 81.1 252.6 30.6 24.2 16.6 38.9 28.7 23.6

a. The sample coefficient of variation assesses the extent of variability relative to the mean (specifically, the standard deviation as a percentage of the mean).

Calculate the coefficient of variation for the sample at each viscosity. Then compare the results and comment.

b. Construct a comparative boxplot of the data and comment on interesting features.