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Unformatted text preview: STAT 409 Examples for 09/28/2009 Fall 2009 The sample proportion : n x p = where x is the number of elements in the sample found to belong to the category of interest (the number of "successes"), and n is the sample size. E( P ) = p , Var( P ) = ( 29 n p p 1 , SD( P ) = ( 29 1 n p p . A largesample confidence interval for the population proportion p is n p p z p  1 2 . 1. Just prior to an important election, in a random sample of 749 voters, 397 preferred Candidate Y over Candidate Z. Construct a 90% confidence interval for the overall proportion of voters who prefer Candidate Y over Candidate Z. Uppertail probability 0.25 0.20 0.15 0.10 0.05 0.025 0.02 0.01 0.005 0.0025 0.001 0.0005 z 0.674 0.841 1.036 1.282 1.645 1.960 2.054 2.326 2.576 2.807 3.091 3.291 50% 60% 70% 80% 90% 95% 96% 98% 99% 99.5% 99.8% 99.9% Confidence level 2. An article on secretaries' salaries in the Wall Street Journal reports: "Threefourth of surveyed secretaries said they make less than $25,000...
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This note was uploaded on 10/12/2011 for the course STATISTICS stat 410 taught by Professor Stepanov during the Spring '11 term at University of Illinois, Urbana Champaign.
 Spring '11
 Stepanov

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