L19-09 - 11/17/09 Lecture 19 1 STOR 155 Introductory...

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Unformatted text preview: 11/17/09 Lecture 19 1 STOR 155 Introductory Statistics Lecture 19: Comparing two proportions Section 8.2 The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL 11/17/09 Lecture 19 2 Two populations: an extension of Lecture 18 Population 1: with proportion Population 2: with proportion Interested in the difference Sample 1: size count proportion Sample 2: size count proportion Consider the difference Assume the two samples are independent , and both and are large . p 1 p 2 p 1 p 2 D = p 1 p 2 p 1 = X 1 =n 1 p 2 = X 2 =n 2 n 1 X 1 n 2 X 2 n 1 n 2 11/17/09 Lecture 19 3 Useful probability facts The random variable D has approximately a normal distribution with mean and standard deviation SD( D ) = An estimate of SD( D ): p 1 p 2 p 1 (1 p 1 ) =n 1 + p 2 (1 p 2 ) =n 2 q SE D = p 1 (1 p 1 ) =n 1 + p 2 (1 p 2 ) =n 2 q 11/17/09 Lecture 19 4 Confidence Interval for Expression: where the margin of error Confidence level C determines m = z SE D z [ D m; D + m ] p 1 p 2 11/17/09...
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This note was uploaded on 11/17/2011 for the course STOR 155 taught by Professor Andrewb.nobel during the Fall '08 term at UNC.

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L19-09 - 11/17/09 Lecture 19 1 STOR 155 Introductory...

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