Montgomery describes an experiment in which the tensile strength of a synthetic fiber is of interest to the manufacturer. It is suspected that strength is related to the percentage of cotton in the fiber. Five levels of cotton percentage are used, and five replicates are run in random order, resulting in the data below. Cotton Percentage 1 2 3 4 5
15 7 7 15 11 9
20 12 17 12 18 18
25 14 18 18 19 19
30 19 25 22 19 23
35 7 10 11 15 11

Required:
a. Does cotton percentage affect breaking strength? Draw comparative box plots and perform an analysis of variance. Use α = 0.05.
b. Plot average tensile strength against cotton percentage and interpret the results.
c. Analyze the residuals and comment on model adequacy.