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Homework Help: Number of samples

  1. Oct 4, 2009 #1
    1. The problem statement, all variables and given/known data

    The determination of iodine in sea water gave a mean value of 46.43 μg/L and a sample standard deviation ss of 1.37 μg/L.

    What is the minimum number of samples (N) which must be analyzed to have 95% confidence that the mean value differs from the true value by no more than 2.0%?

    2. Relevant equations

    u-x(bar)= +/- ts/rt(n) -------> n=(t^2)(s^2)/((u-x)^2)

    3. The attempt at a solution

    I know from the confident interval, the minimum at 95% is 1.96=t, s=1.37, e=0.02,
    where am I going wrong?

    Help please
    Last edited: Oct 4, 2009
  2. jcsd
  3. Oct 4, 2009 #2
    It's been awhile since I've done stats, but doesn't the t value depend on your degrees of freedom, which in turn depends on n?
  4. Oct 5, 2009 #3
    It does but it is not given in the question. I don't know how possible it is to find the degree a freedom from the information given in the question. Any ideas folks?
  5. Oct 5, 2009 #4


    User Avatar

    Staff: Mentor

    For assumed N t is easy to calculate, isn't it? My stats are rusty as hell, but I think you can just prepare a table confidence vs N and check what N value is enough.

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