# Ch08_2 - Chapter 8 Part II Confidence Intervals Chapter 8 1...

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Chapter 8 1 Chapter 8 Part II Confidence Intervals

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Confidence Intervals For population mean Standard deviation known (z) Standard deviation unknown (t) For a proportion (z) Find correct sample size for given error For population mean For proportion
Chapter 8 3 X = monthly cell phone minutes Suppose we don’t know μ . σ = 115.4 A simple random sample with n = 35 had an 553.4 minutes x =

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Chapter 8 4 What do we know? μ -38.24 μ+38.24
Chapter 8 5 CI for population mean Confidence interval estimate for μ : Where α /2 is the upper tail area of the standard normal curve. x z n ± α σ /2 Margin of error

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Chapter 8 6 We can say that has a 95% chance of being within 38.24 minutes of the true value of μ . In other words, 95 out of 100 samples will have a sample mean within 38.24 minutes of μ . x
Chapter 8 7 Regarding this specific sample mean: = 553.4 so we are 95% confident that the true value of μ is between 553.4-38.24 and 553.4+38.24.

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