Name: ______________________________________________
Date: ________________
Confidence Interval Practice Examples
Proportion Examples:
Answer the following using your notes on creating a confidence interval for proportions. Construct
each interval showing all work and interpreting each interval.
1. Currently, at a particular college, seventy percent of college students live in campus
housing. A random sample of 200 students from this college was selected. Create a 99%
confidence interval to estimate the population proportion of all students that live on
campus at colleges of a similar nature.
2. It has been found that, 45% of all first-year students applied to colleges besides the one
they are attending. (This is based on a random sample of 100 first year students).
a. Create a 95% confidence interval to estimate the proportion of all first-year
students that applied to more than one college.
b. A random sample of former Northern Highlands students is selected and it is found
that 85% applied to another school. Do the results from Northern Highlands
students agree with what our population proportion might be?
3. The graph below shows the results of a survey in which 600 adults from Generation X,
600 adults from the baby boomer generation, and 429
senior citizens were asked if they use an investment
professional.
a. Construct a 99% confidence interval for each of
the generations.
b. Based on your calculated intervals, is it possible
that each of the generations could have the
same population proportion that use
investment professionals? Explain.
Review of all procedures:
Determine which of the following is a large sample mean, a small sample mean or a proportion
confidence interval. Then, construct the interval and provide an interpretation.
4. The mean commute to work for 20 randomly selected people was 9.5 miles with a
standard deviation of 2.064. Construct a 90% confidence interval for the mean commute
to work for all working people.
5. In a survey of 2120 college students, 1378 have credit cards. Construct a 99% confidence
interval for the population proportion for all college students that have credit cards.
6. The following data represents time (in minutes) for a random sample of phone calls made
by employees at a company. Construct a 95% confidence interval for the population
mean length of all phone calls made by employees at that company.
7.5
2.0
12.1 8.8
9.4
7.3
1.9
2.8
7.0
7.3