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Math Help - Sample Size

  1. #1
    Junior Member
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    Apr 2008
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    Smile Sample Size

    You want to determine the true mean strength of a new concrete mix with a precision of 50 psi at a confidence level of 99%. If you assume the population standard deviation is 100 psi, what size sample should you use?
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  2. #2
    Member
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    Jul 2008
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    Well the standard deviation of a sample mean is the population standard deviation divided by the square root of the sample size.

    In other words, if \sigma is the population mean of some random variable X, and suppose you are going to take samples of size N. The sample mean, \bar{X} is another random variable. (It's mean is the population mean, but that is not relevant here).

    \sigma_{\bar{X}} = \sigma/\sqrt{N}

    where \sigma_{\bar{X}} is the standard deviation of \bar{X}.

    You know 2 of the 2 variables in that equation so have at it!
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