8. Weld strength experiment, continued For the experiment described in Exercise 7, use the two-way complete model instead of the equivalent cell means model.

(a) Test the hypothesis of no interaction between gage bar setting and time of weld and state your conclusion.

(b) Draw an interaction plot for the two factors Gage bar setting and Time of welding. Does your interaction plot support the conclusion of your hypothesis test? Explain.

(c) In view of your answer to part (b), is it sensible to investigate the differences between the effects of gage bar setting? Why or why not? Indicate on your plot what would be compared.

(d) Regardless of your answer to (c), suppose the experimenters had decided to look at the linear trend in the effect of gage bar settings. Test the hypothesis that the linear trend in gage setting is negligible (against the alternative hypothesis that it is not negligible).