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EX 1

A personnel manager has found that, historically, the scores on aptitude tests given to applicants for entry level positions follow a normal distribution with standard deviation of 32.4 points a random sample of 9 test scores from the current group of applicants had a mean score of 187.9 points.

a. Find the 90% confidence interval for the population mean score of the current group of applicants.

b. Based on these sample results, a statistician found for the population mean, a confidence interval extending from 165.8 to 210.0 points. Find the confidence level for this interval.


EX 2

There is a concern about speed of automobiles travelling over a particular stretch of highway. For a random sample of 7 automobiles, radar indicated the following speeds in miles per hour.

79 73 68 77 86 71 69

Assuming a normal population distribution find the marginal error of a 95% confidence interval for the mean speed of all automobiles travelling over the stretch of highway.


EX 3

Suppose that a random sample of a 600 registered voters in one Florida county, 87.9% strongly agreed with the statement “the state of Florida should have only one method of reading and counting ballots” based on this information, you calculated the confidence interval for the percentage of all registered voters.


EX 4

A company sends a random sample of 12 of its sales people to a course designed to increase their motivation and hence presumably, their effectiveness in the foollowing year, these people generated sales with an average value of $435,000 and a sample standard deviation of $56,000. During the same period, an independently chosen random sample of 15 sales people who had not attended the course obtained sales with an average value of $408,000 and standard deviation $43,000. Assume that, the two population distributions are normal and have the same variance. Find the 95% confidence interval for the difference b etween their means.