Exam 8: Interval Estimation

The average monthly electric bill of a random sample of 256 residents of a city is $90 with a standard deviation of $24. a.Construct a 90% confidence interval for the mean monthly electric bills of all residents. b.Construct a 95% confidence interval for the mean monthly electric bills of all residents.

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

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

A random sample of 49 statistics examinations was taken. The average score, in the sample, was 84 with a variance of 12.25. The 95% confidence interval for the average examination score of the population of the examinations is

When s is used to estimate $\sigma$ , the margin of error is computed by using

Exhibit 8-1 In order to estimate the average time spent on the computer terminals per student at a local university, data were collected for a sample of 81 business students over a one-week period. Assume the population standard deviation is 1.8 hours. -Refer to Exhibit 8-1. With a 0.95 probability, the margin of error is approximately

We are interested in conducting a study in order to determine what percentage of voters of a state would vote for the incumbent governor. What is the minimum size sample needed to estimate the population proportion with a margin of error of 0.05 or less at 95% confidence?

In order to use the normal distribution for interval estimation of $\mu$ when $\sigma$ is known and the sample is very small, the population

The following random sample from a population whose values were normally distributed was collected. The 80% confidence interval for $\mu$ is

It is known that the variance of a population equals 1,936. A random sample of 121 has been taken from the population. There is a .95 probability that the sample mean will provide a margin of error of

The manager of a department store wants to determine what proportion of people who enter the store use the store's credit cards for their purchases. What size sample should he take so that at 95% confidence the error will not be more than 6%?

Exhibit 8-3 The manager of a grocery store has taken a random sample of 100 customers. The average length of time it took these 100 customers to check out was 3.0 minutes. It is known that the standard deviation of the population of checkout times is one minute. -Refer to Exhibit 8-3. With a .95 probability, the sample mean will provide a margin of error of

Which of the following best describes the form of the sampling distribution of the sample proportion?

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

Exhibit 8-2 A random sample of 121 automobiles traveling on an interstate showed an average speed of 65 mph. From past information, it is known that the standard deviation of the population is 22 mph. -Refer to Exhibit 8-2. The 96.6% confidence interval for $\mu$ is

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?

A coal company wants to determine a 95% confidence interval estimate for the average daily tonnage of coal that they mine. Assuming that the company reports that the standard deviation of daily output is 200 tons, how many days should they sample so that the margin of error will be 39.2 tons or less?

In interval estimation, the t distribution is applicable only when

The monthly starting salaries of students who receive an MBA degree have a population standard deviation of $110. What size sample should be selected to obtain a 0.95 probability of estimating the mean monthly income within $20 or less?

A sample of 144 cans of coffee showed an average weight of 16 ounces. The standard deviation of the population is known to be 1.4 ounces. a.Construct a 68.26% confidence interval for the mean of the population. b.Construct a 97% confidence interval for the mean of the population. c.Discuss why the answers in Parts a and b are different.