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	x1x1 = $ 1,101	x2x2 = $ 1,766
Sample Standard Deviation	s1 = $ 696	s2 = $ 838
Repair Incidents	n1 = 12	n2 = 9

Source: Unpublished study by Thomas W. Lauer and Floyd G. Willoughby. (a) Construct a 90 percent confidence 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.)

(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.)

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

multiple choice

• Yes
• No

(d) Construct separate 90% confidence intervals for each mean. (Round your answers to 2 decimal places.)