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Math Help - confidence interval

  1. #1
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    confidence interval

    39 nuts are gathered. They have an average size of 3.34 cent and a standard deviation of .45 cent. Find the 95% confidence interval for the mean size of all the nuts
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  2. #2
    MHF Contributor matheagle's Avatar
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    assuming normality this should be a t with n-1 degrees of freedom,
    which most people will approximate with a z anyhow, which is 1.96 for a 95 percent CI.
    (by the way, is mr fantasy one of the nuts? It would seem so.)

    \bar X \pm t_{n-1,.025}s/\sqrt{n}
    Last edited by mr fantastic; May 17th 2009 at 06:40 AM. Reason: Replaced + - with the latex command for + or -
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  3. #3
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    Quote Originally Posted by matheagle View Post
    assuming normality this should be a t with n-1 degrees of freedom,
    which most people will approximate with a z anyhow, which is 1.96 for a 95 percent CI.
    (by the way is mr fantasy one of the nuts?)
    Can you direct me where to get the t statistic?

    I dont understand where you would use the z value in the formula you gave me
    Last edited by mr fantastic; May 5th 2009 at 05:39 AM. Reason: Added the close quote tag
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  4. #4
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    Quote Originally Posted by mherr View Post
    Can you direct me where to get the t statistic? Mr F says: Use Google eg. 1.3.6.7.2. Upper Critical Values of the Student's-t Distribution

    I dont understand where you would use the z value in the formula you gave me Mr F says: You don't know the population sd, only the sample sd. So the Z-statistic is not used. As baldeagle said, the t-statistic is used. But with df = 38, the critical value of z rather than t can be used without too much error.
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  5. #5
    Junior Member plm2e's Avatar
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    i got the t stat to be 1.94
    Last edited by plm2e; May 5th 2009 at 06:16 AM.
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  6. #6
    MHF Contributor matheagle's Avatar
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    IF we don't have normality, then they want us to use z=1.96 via the central limit theorem. We kind of need to know what the underlying distribution is. Most likley they want the 1.96 percentile from the st normal tables. In any case with n reasonably large (30 according to most books) it doesn't matter which way we go. But one should know where you're sampling from.
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