12.64 In the hope of attracting more riders, a city transit company plans to have express bus service from a suburban terminal to the downtown business district. These buses should save travel time. The city decides to perform a study of the effect of four different plans (such as a special bus lane and traffic signal progression)

on the travel time for the buses. Travel times (in minutes) are measured for several weekdays during a morning rush-hour trip while each plan is in effect. The results are recorded in table below.

a What type of experimental design was employed?
b Is there evidence of a difference in the mean travel times for the four plans? Use 0.01.
c Form a 95% confidence interval for the difference between plan 1 (express lane)
and plan 3 (a control: no special travel arrangements).

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Plan 1 2 3 4 27 2222 25 34 30 25 28 29 33 29 30 32 31 26 27 31 24 36