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Testing of Hypothesis

An experimental bumper was designed to reduce damage in low-speed collisions. This bumper
was installed on an experimental group of vans in a large fleet, but not on a control group. At the
end of a trial period, accident data showed 12 repair incidents for the experimental group and 9 repair
incidents for the control group. Vehicle downtime (in days per repair incident) is shown
below. At ? = .05, did the new bumper reduce downtime?

(a) Make stacked dot plots of the data (a sketch is OK).
(b) State the hypotheses.
(c) State the decision rule and sketch it.
(d) Find the test statistic.
(e) Make a decision.
(f ) Find the p-value and interpret it.
(g) Do you think the difference is large enough to be important? Explain.

(Data are from an unpublished study by Floyd G. Willoughby and Thomas W. Lauer, Oakland University.)
New bumper (12 repair incidents): 9, 2, 5, 12, 5, 4, 7, 5, 11, 3, 7, 1
Control group (9 repair incidents): 7, 5, 7, 4, 18, 4, 8, 14, 13

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