1. A survey of 30 adults found that the mean age of a person’s primary vehicle is 5.6 years.
Assuming the standard deviation of the population is 0.8 year; find the best point estimate
of the population mean and the 99% confidence interval of the population mean.
2. The following data represent a sample of the assets (in millions of dollars) of 30 credit
unions in southwestern Pennsylvania. Find the 90% confidence interval of the mean.
12.23 16.56 4.39
2.89 1.24 2.17
13.19 9.16 1.42
73.25 1.91 14.64
11.59 6.69 1.06
8.74 3.17 18.13
7.92 4.78 16.85
40.22 2.42 21.58
5.01 1.47 12.24
2.27 12.77 2.76
3. A recent study showed that the modern working person experiences an average of 2.1 hours
per day of distractions (phone calls, e-mails, impromptu visits, etc.). A random sample of 50
workers for a large corporation found that these workers were distracted an average of 1.8 hours
per day and the population standard deviation was 20 minutes. Estimate the true mean population
distraction time with 90% confidence.
4. A sample of 80 customers, 60 reply they are satisfied with the service they received. Calculate
a 95% confidence interval for the proportion of satisfied customers
5. A recent study of 28 employees of XYZ Company showed that the mean of the distance they
traveled to work was 14.3 miles. The standard deviation of the sample mean was 2 miles. Find
the 95% confidence interval of the true mean. If a manager wanted to be sure that most of his
employees would not be late, how much time would he suggest they allow for the commute if the
average speed were 30 miles per hour?
6. A medical researcher wishes to determine the percentage of females who take vitamins. He
wishes to be 99% confident that the estimate is within 2 percentage points of the true proportion.
A recent study of 180 females showed that 25% took vitamins. How large should the sample size
be?
Assuming the standard deviation of the population is 0.8 year; find the best point estimate
of the population mean and the 99% confidence interval of the population mean.
2. The following data represent a sample of the assets (in millions of dollars) of 30 credit
unions in southwestern Pennsylvania. Find the 90% confidence interval of the mean.
12.23 16.56 4.39
2.89 1.24 2.17
13.19 9.16 1.42
73.25 1.91 14.64
11.59 6.69 1.06
8.74 3.17 18.13
7.92 4.78 16.85
40.22 2.42 21.58
5.01 1.47 12.24
2.27 12.77 2.76
3. A recent study showed that the modern working person experiences an average of 2.1 hours
per day of distractions (phone calls, e-mails, impromptu visits, etc.). A random sample of 50
workers for a large corporation found that these workers were distracted an average of 1.8 hours
per day and the population standard deviation was 20 minutes. Estimate the true mean population
distraction time with 90% confidence.
4. A sample of 80 customers, 60 reply they are satisfied with the service they received. Calculate
a 95% confidence interval for the proportion of satisfied customers
5. A recent study of 28 employees of XYZ Company showed that the mean of the distance they
traveled to work was 14.3 miles. The standard deviation of the sample mean was 2 miles. Find
the 95% confidence interval of the true mean. If a manager wanted to be sure that most of his
employees would not be late, how much time would he suggest they allow for the commute if the
average speed were 30 miles per hour?
6. A medical researcher wishes to determine the percentage of females who take vitamins. He
wishes to be 99% confident that the estimate is within 2 percentage points of the true proportion.
A recent study of 180 females showed that 25% took vitamins. How large should the sample size
be?
