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Unformatted text preview: Comparing two population proportions Comparing two proportions • Confidence intervals for a difference in proportions • A significance ztest equivalent to the chisquared test for homogeneity Comparing two independent samples We often need to compare two treatments used on independent samples. We can compute the difference between the two sample proportions and compare it to the corresponding, approximately normal sampling distribution for ( 1 – 2 ): p ˆ p ˆ Confidence intervals for two proportions For two independent SRSs of sizes n 1 and n 2 with sample proportion of successes 1 and 2 respectively, an approximate level C confidence interval for p 1 – p 2 is 2 2 2 1 1 1 2 1 ) ˆ 1 ( ˆ ) ˆ 1 ( ˆ * * error of margin the is , ) ˆ ˆ ( n p p n p p z SE z m m m p p diff + = = ± Use this method only when the populations are at least 10 times larger than the samples and the number of successes and the number of failures are each at least 10 in each samples....
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This note was uploaded on 11/05/2011 for the course BMGT 220 taught by Professor Bulmash during the Spring '08 term at Maryland.
 Spring '08
 Bulmash

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