 
Summary: Math 140
Introductory Statistics
Professor Bernardo Ábrego
Lecture 28
Sections 8.4, 8.5, 9.1
Example E65 (page 537)
A poll of 256 boys and 257 girls age 12 to 17 asked, "Do you
feel like you are personally making a positive difference in your
community?" More girls (195) than boys (161) answered "yes."
a. Using a onesided test, is this a statistically significant
difference? That is, if all teens were asked, are you confident
that a larger proportion of girls than boys would say "yes"?
Assume that the samples were selected randomly.
b. The report says, "Participants were selected through
random digit dialing." Do you have any concerns about
whether such a procedure would give a random sample?
c. Find a 95% confidence interval for the proportion of all
teens who would answer yes. What additional assumption
do you need to make to do this?
9.1 A Confidence Interval for a Mean
