Lecture 8 Part I.pdf


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Confidence Interval: Interpretation
If we select again and again many sample from the population,
and for each one, we compute the sample mean, and create an
interval ranging from

Common “Z” levels of confidence
• Commonly used confidence levels are 90%,
95%, and 99%
Confidence
Level

that (1 - α) % of the times the interval contains the population
mean µ.
• The value of Z is decided by the desired level of confidence.
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Intervals & Level of Confidence
Sampling

80%
90%
95%
98%
99%
99.8%
99.9%

Z value
1.28
1.645
1.96
2.33
2.58
3.08
3.27

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95% Confidence Interval
• For 95% confidence, α = 0.05 and α / 2 = 0.025.
• The value of Z0.025 is found by looking in the standard
normal table under 0.975.
• This area in the table is associated with a Z value of
1.96.

Confidence Intervals

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