Exam 8: Interval Estimation

As the degrees of freedom increase, the t distribution approaches the

A researcher is interested in determining the average number of years employees of a company stay with the company. If past information shows a standard deviation of 7 months, what size sample should be taken so that at 95% confidence the margin of error will be 2 months or less?

In determining the sample size necessary to estimate a population proportion, which of the following information is not needed?

If the margin of error in an interval estimate of  is 4.6, the interval estimate equals

A manufacturer wants to estimate the proportion of defective items that are produced by a certain machine. A random sample of 50 items is taken. Which Excel function would not be appropriate to construct a confidence interval estimate?

A simple random sample of 144 items resulted in a sample mean of 1080. The population standard deviation is known to be 240. Develop a 95% confidence interval for the population mean.

A random sample of 25 observations was taken from a normally distributed population. The average in the sample was 84.6 with a variance of 400. a.Construct a 90% confidence interval for . b.Construct a 99% confidence interval for . c.Discuss why the 90% and 99% confidence intervals are different. d.What would you expect to happen to the confidence interval in part a if the sample size was increased? Be sure to explain your answer.

From a population that is not normally distributed and whose standard deviation is not known, a sample of 50 items is selected to develop an interval estimate for . Which of the following statements is true?

Fifty students are enrolled in an Economics class. After the first examination, a random sample of 5 papers was selected. The grades were 60, 75, 80, 70, and 90. a.Calculate the estimate of the standard error of the mean. b.What assumption must be made before we can determine an interval for the mean grade of all the students in the class? Explain why. c.Assume the assumption of Part b is met. Provide a 90% confidence interval for the mean grade of all the students in the class. d.If there were 200 students in the class, what would be the 90% confidence interval for the mean grade of all the students in the class?

As the sample size increases, the margin of error

Below you are given ages that were obtained by taking a random sample of 9 undergraduate students. Use Excel to determine an interval estimate for the mean of the population at a 99% confidence level. Interpret your results.

The confidence associated with an interval estimate is called the

A simple random sample of 25 items from a normally distributed population resulted in a sample mean of 28 and a standard deviation of 7.5. Construct a 95% confidence interval for the population mean.

If we change a 95% confidence interval estimate to a 99% confidence interval estimate, we can expect the

An estimate of a population parameter that provides an interval believed to contain the value of the parameter is known as the

To compute the minimum sample size for an interval estimate of , we must first determine all of the following except

For inventory purposes, a grocery store manager wants to estimate the mean number of pounds of cat food sold per month. The estimate is desired to be within 10 pounds with a 95% level of confidence. A pilot study provided a standard deviation of 27.6 pounds. How many months should be sampled?

An interval estimate is used to estimate

The use of the normal probability distribution as an approximation of the sampling distribution of is based on the condition that both np and n(1 - p) equal or exceed

A random sample of 81 students at a local university showed that they work an average of 100 hours per month. The population standard deviation is known to be 27 hours. Compute a 95% confidence interval for the mean hours per month all students at the university work.