Question: In Dallas , some fire trucks were painted yellow (instead of red) to heighten their visibility. During a test period, the fleet of red fire trucks made 153,348 runs and had 20 accidents, while the fleet of yellow fire trucks made 135,035 runs and had 4 accidents. At α = .01, did the yellow fire trucks have a significantly lower accident rate?

1. State the hypotheses.
2. State the decision rule and sketch it.
3. Find the sample proportions and z test statistic.
4. Make a decision.
5. Find the p -value and interpret it.
6. If statistically significant, do you think the difference is large enough to be important? If so, to whom, and why? (g) Is the normality assumption fulfilled? Explain.

Source: The Wall Street Journal, June 26, 1995 , p. B1.

Accident Rate for Dallas Fire Trucks

Statistic Red Fire Trucks Yellow Fire Trucks

Number of accidents x 1 = 20 accidents x 2 = 4 accidents

Number of fire runs n 1 = 153,348 runs n 2 = 135,035 runs

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