 
Summary: Math 140
Introductory Statistics
Professor Bernardo Ábrego
Lecture 22
Sections 8.1
Back to Opinion Polls.
At the beginning of this section, you read about a recent Phi
Delta Kappa/Gallup poll that reported that a record 51% of the
American public assigns a grade of A or B to the public schools
in their community. The sample size was 1108. This survey had
a margin of error of 3%, and so their 95% confidence interval is
48% to 54%. (The procedure Gallup uses to select a sample is
more complicated than simple random sampling, but you can
use your formula for a confidence interval to approximate
Gallup's margin of error.)
You now should be able to answer these questions:
What is it that you are 95% sure is in the confidence interval?
Answer: The proportion of all Americans who would assign a
grade of A or B to their local public schools.
Back to Opinion Polls.
