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 1-year period. Those with the special bumper are the test group and the other vehicles are the control group, shown below. Each "repair incident" is defined as an invoice (which might include more than one separate type of damage).

Statistic Test Group Control Group

Mean Damage X1 = $ 1,153 X2 = $ 1,751

Sample Std. Dev. s1 = $ 663 s2 = $ 820

Repair Incidents n1 = 16 n2 = 14

(a)Construct a 99 percent confidence interval for the true difference of the means assuming equal variances.(Round your final answers to 3 decimal places.Negative values should be indicated by a minus sign.)

The 99% confidence interval is from__________to__________

(b)Repeat part (a), using the assumption of unequal variances with Welch's formula ford.f.(Round the calculation for Welch's df to the nearest integer. Round your final answers to 3 decimal places. Negative values should be indicated by a minus sign.)

The 99% confidence interval is from ________to_________

(c)Did the assumption about variances change the conclusion?

Yes

No

(d)Construct separate 99% confidence intervals for each mean.(Round your intermediatet_critvalue to 3 decimal places. Round your final answers to 2 decimal places.)

Mean Damage Confidence Interval

x1=$1,153 ($_______, $________)

x2=$1,751 ($_______, $________)