STAT 30100 Elementary Statistical Methods
Summer 2026 Online
Confidence Intervals
1. The average annual dollar amount 1601 families paid for college was
$20,902. If the population standard deviation is $7500, give the 95%
confidence interval for the mean.
2. Since 2007, the American Psychological Association has supported an
annual nationwide survey to examine stress across the US. A total of 340
adults, ages 18 to 33 years old, were asked to indicate their average stress
level on a 10 point scale. The mean score was 5.4. If we assume a
population standard deviation of 2.3, calculate the confidence interval for a
99% confidence.
3. You are planning a survey of starting salaries for recent CNIT graduates.
According to your research, the average starting salary is $60,038 with a
population standard deviation of $4300. What sample size would you need
to have a margin of error of $500 at a 95% confidence?
4. A bone study that measures a biomarker, TRAP, yields a mean of 13.2
units/liter. Assuming the population standard deviation is 6.5 units/liter, what
sample size would be required at a 95% confidence to get a margin of error
of 1.5 units/liter?
5. Use the “Confidence Simulator” link on Brightspace. We are going to assume
that the data you are using is scores on a statistics final exam in a traditional
course. Set the Population Mean to 55 and the Population SD to 12. Then
use the settings below to answer the following questions.
a. Set Confidence Level to 80, sample size of 20; take 500 samples and
note how many Confidence Intervals HIT.
b. Set Confidence Level to 90, sample size to 20; take 500 samples and
note how many Confidence Intervals HIT.
c. Set Confidence Level to 90, sample size to 100; take 500 samples
and note how many Confidence Intervals HIT.
d. Compare the above results and state what you notice.
6. Answer the following questions from your textbook (Note: page numbers are
from paperback print version and your page numbers may be different;
problem numbers, however, are the same in all versions): p.280 – Exercise 5.3
& 5.6; p.296 – Exercise 5.16; p.343 – 6.1.
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