Math 140 Confidence Intervals Activity #8
Calculating Confidence Intervals with Statcrunch, Minitab or Statcato
Confidence intervals can be constructed with Statistics software like Statcrunch, Minitab and
Statcato.
Statcrunch: To construct a confidence interval for one proportion, go to “Stat” tab, then
“Proportion stats”, then “One Sample” and “with summary”. Plug in the number of events and
trials and choose “confidence interval”. To construct a confidence interval for the mean with
the z-score standard normal curve, go to “Stat” tab, then “Z-stats”, then “One Sample” and
“with summary”. Plug in the sample mean, sample standard deviation and sample size and
choose “confidence interval”. (Note: We will see later that we may choose to use the tdistribution instead of the z-score).
Statcato: To construct a confidence interval on Statcato, go to the “Statistics” tab, then to
“Confidence Intervals”. Then click on the type of confidence interval you want (1 proportion or
1 mean). Under summary statistics enter the information required.
(#1-3) Directions: Use Statcrunch, Minitab or Statcato to calculate the confidence interval
estimate of the population mean  . Then write a sentence that explains the meaning of the
confidence interval in the context of the problem.
1. A random sample of 650 high school students has a normal distribution. The sample
mean average ACT exam score was 21 with a 3.2 sample standard deviation. Construct
a 90% confidence interval estimate of the population mean average ACT exam. Also
construct a 95% confidence interval estimate of the population mean average ACT exam.
2. A random sample of 200 adults found that they had a sample mean temperature of 98.2
degrees and a standard deviation of 0.18 degrees. Construct a 95% confidence interval
estimate of the population mean body temperature of adults. Construct a 99% confidence
interval estimate also. Do the confidence intervals indicate that normal body temperature
could be 98.6 degrees?
3. A random sample of 315 adults found that the sample mean amount or credit card debt
was $435 with a standard deviation of $106. Construct a 90% confidence interval
estimate of the population mean amount of credit card debt. Also construct a 95%
confidence interval estimate of the population mean amount of credit card debt.
(#4-6) Directions: Use Statcrunch, Minitab or Statcato to calculate the confidence interval
estimate of the population percent p . Then write a sentence that explains the meaning of
the confidence interval in the context of the problem. You may have to use the formula
x  pˆ n if the number of events is not given.
4. What percent of eligible Americans vote? In 2008, a random sample of 3000 American
adults was taken and we found that 68% of them voted. Construct a 90% confidence
interval estimate of the population percent of Americans that vote. Now construct a 99%
confidence interval estimate.
5. In a random sample of 72 adults in Santa Clarita, CA, each person was asked if they
support the death penalty. 31 adults in the sample said that they do support the death
penalty. What was the sample proportion of adults in Santa Clarita that support the death
penalty? What is the standard error for this sampling distribution? Now calculate a 95%
confidence interval population estimate of people in Santa Clarita that support the death
penalty. Then calculate a 90% confidence interval estimate of the population.
6. In a random sample of 400 Americans, each person was asked if they are satisfied with
the amount of vacation time they given by their employers. 16% of them said that they
were satisfied and 84% of them said they were not satisfied. Calculate the following.
What was the sample proportion that was satisfied with their vacation time? What was
the standard error for the sampling distribution? Now calculate a 99% confidence
interval population estimate of Americans that are satisfied with their vacation time and a
99% confidence interval population estimate of Americans that are not satisfied with their
vacation time.
7. Answer the following questions.
a. As the confidence level gets higher, does the confidence interval get narrower
or wider? Why?
b. As the confidence level decreases, what happens to the margin of error? Why?
c. As the sample size increases, what happens to the standard error? Why?