Cell phone use by drivers. In a July 2001research note, the U.S. Department of T

Question

Cell phone use by drivers. In a July 2001research note, the

U.S. Department of Transportation reported the results of

the National Occupant Protection Use Survey. One focusof

the survey was to determine the level of cell phone use by

drivers while they are in the act of driving a motorpassenger

vehicle. Data collected by observers at randomly

selected intersections across the country revealed that in a

sample of 1,165 drivers, 35 were using their cell phone.

a. Give a point estimate of p, thetrue driver cell phone use

rate (i.e., the true proportion of drivers who are using a

cell phone while driving)

b. Compute a 95% confidence interval forp.

c. Give a practical interpretation of theinterval, part b.

Explanation / Answer

Given n=1165, (a)p=35/1165=0.03 (b) Given =0.05, |Z(0.025)|=1.96 (check normal table) The 95% CI is p ± Z*p*(1-p)/n --> 0.03 ± 1.96*sqrt(0.03*(1-0.03)/1165) --> (0.02, 0.04) (c) We have 95% confidence that p will be in this interval.

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