The postal service sorts mail as Priority Mail Express, Priority Mail, First-Class Mail, or Standard Mail. Over a period of 3 weeks, 18 of each type were mailed from the Network Distribution Center in Altanta, Georgia, to Des Moines, Iowa. The total delivery time in days was recorded. Minitab was used to perform the ANOVA. The results follow: Source DF SS MS F P
Factor 3 1.92 0.64 2.91 0.041
Error 68 14.77 0.22
Total 71 16.69

Level N Mean StDev
Priority Mail Express 18 2.773 0.543
Priority Mail 18 3.112 0.561
First-Class Mail 18 3.183 0.333
Standard Mail 18 3.141 0.385

Using the ANOVA results, compare the average delivery times of the four different types of mail.

a. Identify the null hypothesis and the alternate hypothesis.
b. What is the decision rule?