Exam 8: Interval Estimation

A survey of 40 students at a local college asks, "Where do you buy the majority of your books?" The responses fell into three categories: "at the campus bookstore," "on the Internet," and "other." The results follow. Use Excel to estimate the proportion of all of the college students who buy their books on the Internet.

We can reduce the margin of error in an interval estimate of p by doing any of the following except

A small stock brokerage firm wants to determine the average daily sales (in dollars) of stocks to their clients. A sample of the sales for 36 days revealed average daily sales of $200,000. Assume that the standard deviation of the population is known to be $18,000. a.Provide a 95% confidence interval estimate for the average daily sale. b.Provide a 97% confidence interval estimate for the average daily sale.

The makers of a soft drink want to identify the average age of its consumers. A sample of 16 consumers is taken. The average age in the sample was 22.5 years with a standard deviation of 5 years. Assume the population of consumer ages is normally distributed. a.Construct a 95% confidence interval for the average age of all the consumers. b.Construct an 80% confidence interval for the average age of all the consumers. c.Discuss why the 95% and 80% confidence intervals are different.

The manager of a department store wants to determine what proportion of people who enter the store use the store's credit card for their purchases. What size sample should he take so that at 99% confidence the error will not be more than 8%?

Exhibit 8-3 A random sample of 81 automobiles traveling on a section of an interstate showed an average speed of 60 mph. The distribution of speeds of all cars on this section of highway is normally distributed, with a standard deviation of 13.5 mph. -Refer to Exhibit 8-3. If the sample size was 25 (other factors remain unchanged), the interval for  would

It is known that the variance of a population equals 484. A random sample of 81 observations is going to be taken from the population. A)With a .80 probability, what statement can be made about the size of the margin of error? B)With a .80 probability, how large of a sample would have to be taken to provide a margin of error of 3 or less?

A sample of 16 students from a large university is taken. The average age in the sample was 22 years with a standard deviation of 6 years. Construct a 95% confidence interval for the average age of the population. Assume the population of student ages is normally distributed.

A sample of 26 elements from a normally distributed population is selected. The sample mean is 10 with a standard deviation of 4. The 95% confidence interval for  is

The z value for a 97.8% confidence interval estimation is

A random sample of 25 employees of a local company has been measured. A 95% confidence interval estimate for the mean systolic blood pressure for all company employees is 123 to 139. Which of the following statements is valid?

You are given the following information obtained from a random sample of 4 observations from a large, normally distributed population. a.What is the point estimate of ? b.Construct a 95% confidence interval for . c.Construct a 90% confidence interval for . d.Discuss why the 90% and 95% confidence intervals are different.

The average monthly electric bill of a random sample of 256 residents of a city is $90. The population standard deviation is assumed to be $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.

Exhibit 8-1 In order to estimate the average time spent on the computer terminals per student at a local university, data were collected from a sample of 81 business students over a one-week period. Assume the population standard deviation is 1.2 hours. -Refer to Exhibit 8-1. With a 0.95 probability, the margin of error is approximately

Exhibit 8-3 A random sample of 81 automobiles traveling on a section of an interstate showed an average speed of 60 mph. The distribution of speeds of all cars on this section of highway is normally distributed, with a standard deviation of 13.5 mph. -Refer to Exhibit 8-3. The value to use for the standard error of the mean is

A local university administers a comprehensive examination to the recipients of a B.S. degree in Business Administration. A sample of 5 examinations is selected at random and scored. The scores are shown below. Use Excel to determine an interval estimate for the mean of the population at a 98% confidence level. Interpret your results.

A sample of 100 cans of coffee showed an average weight of 13 ounces. The population standard deviation is 0.8 ounces. a.Construct a 95% confidence interval for the mean of the population. b.Construct a 95.44% confidence interval for the mean of the population. c.Discuss why the answers in parts a and b are different.

Exhibit 8-3 A random sample of 81 automobiles traveling on a section of an interstate showed an average speed of 60 mph. The distribution of speeds of all cars on this section of highway is normally distributed, with a standard deviation of 13.5 mph. -Refer to Exhibit 8-3. If we are interested in determining an interval estimate for  at 86.9% confidence, the z value to use is

For the interval estimation of  when  is assumed known, the proper distribution to use is the

Exhibit 8-2 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 checkout time is one minute. -Refer to Exhibit 8-2. The standard error of the mean equals