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Question: a special bumper was installed on selected vehicles in a...

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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⎯⎯⎯1X1 = $ 1,172 X⎯⎯⎯2X2 = $ 1,740
  Sample Std. Dev. s1 = $ 668 s2 = $ 808
  Repair Incidents n1 =    15 n2 =    12
Source: Unpublished study by Thomas W. Lauer and Floyd G. Willoughby.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 intermediate tcrit value to 3 decimal places. Round your final answers to 3 decimal places. Negative values should be indicated by a minus sign.)

  The 90% confidence interval is from  to
(b)

Repeat part (a), using the assumption of unequal variances with Welch's formula for d.f. (Round your intermediate tcrit value to 3 decimal places. Round your final answers to 3 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?
Yes
No
(d) Construct separate 90% confidence intervals for each mean. (Round your intermediate tcrit value to 3 decimal places. Round your final answers to 2 decimal places.)
Mean Damage Confidence Interval
   x⎯⎯1=$1,172x1$1,172 ($ , $ )  
   x⎯⎯2=$1,740x2$1,740 ($ , $
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