A special bumper was installed on selected vehicles in a large fleet. The dollar cost of body repairs was recorded for all vehicles that were involved in accidents over a 1year period. Those with the special bumper are the test group and the other vehicles are the control group. shown below. Each "repair incident" is dened as an invoice (which might include more than one separate type of damage). Statistic Test Group Control Grou Mean Damage 5'1 2 $ 1,181 h = $ 1,?! Sample Standard Deviation 51 = $ 595 52 = $ 33 Repair Incidents n1 = 12 n2 = Source: Unpublished study by Thomas W. Lauer and Floyd G. Willoughbv. [a] Construct a 90 percent condence interval for the true difference of the means assuming equal variances. (Round your answers to 3 decimal places. Negative values should be indicated by a minus sign.) The 90% condence interval is from I I to I I (b) Repeat part (a), using the assumption of unequal variances with Welch's (b) Repeat part (a), using the assumption of unequal variances with Welch's formula for d.f. (Round your answers to 2 decimal places. Negative values should be indicated by a minus sign.) The 90% confidence interval is from to (c) Did the assumption about variances change the conclusion? O Yes O No (d) Construct separate 90% confidence intervals for each mean. (Round your answers to 2 decimal places.) Mean Damage Confidence Interval x bar, = $1, 101 x bar2 = $1,766