An operations manager wants to examine the effect of air-jet pressure (in pounds per square inch [psi]) on the breaking strength of yarn. Three different levels of air-jet pressure are to be considered: 30 psi, 40 psi, and 50 psi. A random sample of 18 yarns are selected from the same batch, and the yarns are randomly assigned, 6 each, to the 3 levels of air-jet pressure. The breaking strength scores are in the file Yarn.
a. Is there evidence of a significant difference in the variances of the breaking strengths for the three air-jet pressures? (Use a = 0.05.)
b. At the 0.05 level of significance, is there evidence of a difference among mean breaking strengths for the three air-jet pressures?
c. If appropriate, use the Tukey-Kramer procedure to determine which air-jet pressures significantly differ with respect to mean breaking strength. (Use a = 0.05.)
d. What should the operations manager conclude?