Elementary Statistics: A Step-by-Step Approach with Formula Card 9th Edition

Published by McGraw-Hill Education
ISBN 10: 0078136334
ISBN 13: 978-0-07813-633-7

Chapter 7 - Confidence Intervals and Sample - 7-3 Confidence Intervals and Sample Size for Proportions - Exercises 7-3 - Page 395: 6

Answer

$0.233\lt\mu\lt0.401$

Work Step by Step

1. We have $\hat p=\frac{65}{205}=0.317, \hat q=1-\hat p=0.683 , n=205 $ 2. At a 99% confidence the critical z-value is $z_{\alpha/2}=2.575$ 3. The margin of error can be found as $E=z_{\alpha/2}\times\sqrt {\frac{\hat p\hat q}{n}}=2.575\times\sqrt {\frac{0.317\times0.683}{205}}=0.084$ 4. Thus, the interval of the true proportion can be found as $\hat p-E\lt p\lt\hat p+E$ which gives $0.233\lt\mu\lt0.401$ 5. The number from the Time magazine, $37\%$, is in the interval.
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