Researchers Olivia Carrillo-Gamboa and Richard Gunst presented the following data in their article, "Measurement-Error-Model Collinearities." The data represent the compressive strength (in pounds per square inch) of a random sample of concrete 2 days, 7 days, and 28 days after pouring.

(a) Normal probability plots for each treatment indicate that the requirement of normality is satisfied. Verify that the requirement of equal population variances for each treatment is satisfied.
(b) Is there sufficient evidence that the mean strength is different among the three days at the a = 0.05 level of significance?
(c) If the null hypothesis from part (b) was rejected, use Tukey's test to determine which pairwise means differ using a familywise error rate of a = 0.05.

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7 Days 28 Days Concrete 2 Days 3505 2830 4470 3295 3430 4740 2710 3670 5115 2855 3355 4880 2980 3985 4445 3065 3630 4080 3765 4570 5390