Exam 8: Interval Estimation

It is known that the population variance equals 484. With a 0.95 probability, the sample size that needs to be taken if the desired margin of error is 5 or less is

Whenever using the t distribution for interval estimation (when the sample size is very small), we must assume that

An Excel printout of the descriptive measures of daily checking account balances (in dollars) of customers of First Daisy Bank is shown below. Develop a 95% confidence interval estimate for the mean of the population of the checking balances.

A sample of 144 commuters in Boston indicated that they commute an average of one hour and thirty minutes per day. The standard deviation of the sample was 36 minutes. Compute the 95% confidence interval for the average commute time of the population (in minutes).

The sample size needed to provide a margin of error of 2 or less with a .95 probability when the population standard deviation equals 11 is

Exhibit 8-4 In order to estimate the average electric usage per month, a sample of 81 houses was selected, and the electric usage was determined. Assume a population standard deviation of 450-kilowatt hours. -Refer to Exhibit 8-4. At 95% confidence, the size of the margin of error is

The makers of a soft drink want to identify the average age of its consumers. A sample of 55 consumers was taken. The average age in the sample was 21 years with a standard deviation of 4 years. a.Construct a 95% confidence interval for the true average age of the consumers. b.Construct an 80% confidence interval for the true average age of the consumers. c.Discuss why the 95% and 80% confidence intervals are different.

A random sample of 121 checking accounts at a bank showed an average daily balance of $280. The standard deviation of the population is known to be $66. a.Is it necessary to know anything about the shape of the distribution of the account balances in order to make an interval estimate of the mean of all the account balances? Explain. b.Find the standard error of the mean. c.Give a point estimate of the population mean. d.Construct a 80% confidence interval estimates for the mean. e.Construct a 95% confidence interval for the mean.

A sample of 36 patients in a doctor's office showed that they had to wait an average of 45 minutes with a standard deviation of 10 minutes before they could see the doctor. Provide a 90% confidence interval estimate for the average waiting time of all the patients who visit this doctor.

A random sample of 64 students at a university showed an average age of 25 years and a sample standard deviation of 2 years. The 98% confidence interval for the true average age of all students in the university is

In order to determine the average weight of carry-on luggage by passengers in airplanes, a sample of 16 pieces of carry-on luggage was weighed. The average weight was 20 pounds. Assume that we know the standard deviation of the population to be 8 pounds. a.Determine a 97% confidence interval estimate for the mean weight of the carry-on luggage. b.Determine a 95% confidence interval estimate for the mean weight of the carry-on luggage.

A random sample of 49 lunch customers was taken at a restaurant. The average amount of time the customers in the sample stayed in the restaurant was 45 minutes with a standard deviation of 14 minutes. a.Compute the standard error of the mean. b.With a .95 probability, what statement can be made about the size of the margin of error? c.Construct a 95% confidence interval for the true average amount of time customers spent in the restaurant. d.With a .95 probability, how large of a sample would have to be taken to provide a margin of error of 2.5 minutes or less?

Exhibit 8-6 A sample of 75 information system managers had an average hourly income of $40.75 with a standard deviation of $7.00. -Refer to Exhibit 8-6. The 95% confidence interval for the average hourly wage of all information system managers is

The Highway Safety Department wants to study the driving habits of individuals. A sample of 121 cars traveling on the highway revealed an average speed of 60 miles per hour with a standard deviation of 11 miles per hour. Determine a 95% confidence interval estimate for the speed of all cars.

Six hundred consumers were asked whether they would like to purchase a domestic or a foreign automobile. Their responses are given below. Develop a 95% confidence interval for the proportion of all consumers who prefer to purchase domestic automobiles.

A population has a standard deviation of 50. A random sample of 100 items from this population is selected. The sample mean is determined to be 600. At 95% confidence, the margin of error is

A sample of 200 elements from a population with a known standard deviation is selected. For an interval estimation of $\mu$ , the proper distribution to use is the

Information regarding the price of a roll of camera film (35 mm, 24 exposure) for a sample of 12 cities worldwide is shown below. Determine a 95% confidence interval for the population mean.

Whenever the population standard deviation is unknown and the population has a normal or near-normal distribution, which distribution is used in developing an interval estimation?

In a poll of 600 voters in a campaign to eliminate non-returnable beverage containers, 210 of the voters were opposed. Develop a 92% confidence interval estimate for the proportion of all the voters who opposed the container control bill.