15 In a study of 200 accidents that required treatment in an

15. In a study of 200 accidents that required treatment in an emergency room, 40% occurred at home. Find a 90% confidence interval for the true proportion of accidents that occur at home.

14. The heights of a random sample of 38 male officers from a large-city police force were measured. The standard deviation for the sample was 1.83 inches. Find a 95% confidence interval for the standard deviation of the heights of the officers. Heights of men are known to be normally distributed.

13. A simple random sample of textbooks in a local college bookstore yielded a mean textbook cost of $74.80 with a standard deviation of $19.79. Find an 85% confidence interval for the true mean cost of a college textbook.

Solution

15) Proportion for accidents = 40% = 0.40

Var of p = p(1-p)/n

z alpha /2 = 1.645

Sample Size: 200
Observed Proportion: 40%
Confidence Level: 90%

Confidence Interval:

±5.7

Range for the true population proportion:

34.3% to 45.7%

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Results

Confidence Interval:

±0.2

Range for the true population mean:

74.6 to 75