Hypothesis testing-difference-two population proportions


In the Dallas, some fire trucks were painted yellow (instead of the red colour) to heighten their visibility. Throughout 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 the significantly lower accident rate?

(a) State hypotheses.

(b) State decision rule and sketch it.  

(c) Find out the sample proportions and test statistic.   

(d) Make a decision.

(e) Find out the p-value and interpret it.  

(f ) If statistically significant, do you think the difference is large enough to be significant? If so, to whom, and why?

(g) Is the normality assumption fulfilled? Describe.

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              x1 = 20 accidents                    x2 = 4 accidents

Number of fire runs                n1 = 153,348 runs                   n2 = 135,035 runs

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Basic Statistics: Hypothesis testing-difference-two population proportions
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