Chapter 8 Confidence Intervals

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Chapter 8Confidence Intervals

• 8.3
• Confidence Intervals about a Population Proportion

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EXAMPLE Computing a Point Estimate In a Fox
News Poll conducted January 27-28, 1999, 911
registered voters nationwide were asked Have you
ever cheated on a person you were in a
relationship with? Of the 911 respondents, 191
said yes. Obtain a point estimate for the
proportion of registered voters that have cheated
on a person they were in a relationship with.
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EXAMPLE Constructing a Confidence Interval
for a Population Proportion In a Fox News Poll
conducted January 27-28, 1999, 911 registered
voters nationwide were asked Have you ever
cheated on a person you were in a relationship
with? Of the 911 respondents, 191 said yes.
Compute a 95 confidence interval for the
proportion of registered voters that have cheated
on a person they were in a relationship with.
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Sample Size Needed for a Specified Margin of
Error and Level of Confidence
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EXAMPLE Determining Sample Size A sociologist
wanted to determine the percentage of residents
of America that only speak English at home. What
size sample should be obtained if she wishes the
estimate to be within 3 percentage points with
90 confidence assuming (a) she uses the 2000
estimate obtained from the Census 2000
Supplementary Survey of 82.4. (b) she does not
use any prior estimates.