In Dallas some fire trucks were painted yellow9instead of red) to heighten their
ID: 2915062 • Letter: I
Question
In Dallas some fire trucks were painted yellow9instead of red) to heighten their visibility. During a testperiod, the fleet red of fire trucks made 153,348 runs and had 20accidents, while the fleet of yellow fire trucks made 135,035 runsand had 4 accidents. At a = .01, did the yellow fire trucks have asignificantly lower accident rate?
a)STate the hypothesis.
b) State the decision rule and sketchit.
c) Find the sample proportions and z teststatistic.
d) Make a decision.
e) Find the p-value and intrepret it.
f) If statistically significant, do you think thedifference is large enough to be important? if so, to whom, andwhy?
g) Is the normality assumption fulfilled?Explain.
Accident Rate for Dallas Firetrucks
Statistic RedFire Trucks Yellow firetrucks
# of accidents x1=20accidents x2= 4accidents
# of fireruns n1= 153, 348runs n2= 135,035 runs
Explanation / Answer
b. Reject the null hypothesis ifz > 2.326.
d. We reject the null hypothesis. Thesample evidence shows a significant difference in the twoproportions.
e. The p-value = .0031. Thisresult is not due to chance.
f. Yes, accidents have severeconsequences therefore even small reductions make a difference.
g. The normality assumption isquestionable because there were only 4 accidents observed with theyellow fire trucks.