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Department store chain, confidence interval for mean

1. A major department store chain is interested in estimating the average amount its credit card customers spent on their first visit to the chain's new store in the mall. Fifteen credit card accounts were randomly sampled and analyzed with the following results:
Χ = $50.50 and s2 = 400.

Assuming the distribution of the amount spent on their first visit is approximately normal, what is the shape of the sampling distribution of the sample mean that will be used to create the desired confidence interval for μ?

a. Approximately normal with a mean of $50.50
b. A standard normal distribution
c. A t distribution with 15 degrees of freedom
d. A t distribution with 14 degrees of freedom

2. The personnel director of a large corporation wishes to study absenteeism among clerical workers at the corporation's central office during the year. A random sample of 25 clerical workers revealed the following:
? Absenteeism: X = 9.7 days and S = 4.0 days
? 12 clerical workers were absent more than 10 days

What are the upper and lower limits of the 95% confidence interval estimate of the mean number of absences for clerical workers last year?

a. 8.132 and 11.268
b. 8.049 and 11.351
c. 8.052 and 11.348
d. 8.331 and 11.069

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The solution gives the details of calculation of 95% confidence interval of mean.

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