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 Atlanta, 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 2.45 0.82 2.93 0.040
Error 68 19.11 0.28
Total 71 21.56

Level N Mean StDev
Priority Mail Express 18 2.959 0.397
Priority Mail 18 3.466 0.714
First-Class Mail 18 3.163 0.606
Standard Mail 18 3.106 0.300

Using the ANOVA results, compare the average delivery times of the four different types of mail. Identify the null hypothesis and the alternate hypothesis.