Question

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	X¯¯¯1X¯1	=	$	1,157		X¯¯¯2X¯2	=	$	1,775
Sample Std. Dev.	s1	=	$	662		s2	=	$	827
Repair Incidents	n1	=		17		n2	=		13

Source: Unpublished study by Thomas W. Lauer and Floyd G. Willoughby.

(a) Construct a 98 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 98% confidence interval is from ___ to ___

(b) Repeat part (a), using the assumption of unequal variances with Welch's formula for d.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 98% confidence interval is from ___ to ___

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

• Yes

• No

(d) Construct separate 98% confidence intervals for each mean. (Round your intermediate t_crit value to 3 decimal places. Round your final answers to 2 decimal places.)

Mean Damage	Confidence Interval
x1=$1,157	($___  , $ ___ )
x2=$1,775	($ ___ , $ ___ )

Answer 1