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Math Help - formula for confidence interval for a mean

  1. #1
    Member Jskid's Avatar
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    [RESOLVED]formula for confidence interval for a mean

    In the following formula for creating a confidence interval for a mean where do I get Z_{\alpha/2} from? \bar{X}\pm Z_{\alpha/2}\frac{\sigma}{\sqrt{n}}

    I know it has something to do with using z-scores and I've been trying to work backwards from examples. For example a confidence interval of 90% should have Z_{\alpha/2}=1.645 but when I look up 0.9 I get 0.8159.
    Last edited by Jskid; August 5th 2010 at 03:47 PM. Reason: resolved
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  2. #2
    Master Of Puppets
    pickslides's Avatar
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    Z table - Normal Distribution

    Go to the second graph and type in 0.9 in 'shaded area' and click on 'between' as \frac{\alpha}{2} implies a two tail interval
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