Confidence Intervals for Proportions

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• Confidence Intervals for
  Proportions

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The Random Variable

• Normal distribution
• Mean ? p
• Standard Deviation

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Confidence Intervals For p

• (Point Estimate) ? z?/2(Appropriate Std
  Deviation)
• Suppose there are x successes in a sample of size
  n
• POINT ESTIMATE
• APPROPRIATE STD DEV.
• APPROXIMATE BY
• CONFIDENCE INTERVAL

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EXAMPLE

• Taste test between Coke and Pepsi
• 1000 cola drinkers tested
• 540 say they prefer Coke
• Give a 95 confidence interval for the proportion
  of cola drinkers who favor Coke.
• Note

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Confidence Interval For p The Proportion That
Favor Coke

• 95 confidence interval

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Constructing Confidence Intervals for p Using
Excel

• Suppose cell C5 contains n, cell C7 contains a
  and cell C9 contains
• CONFIDENCE INTERVAL

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Review

• The point estimate for p is
• The confidence interval for p is
• Use of Excel (COUNTA, COUNTIF Functions)