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Math Help - Sample variance equation question

  1. #1
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    Sample variance equation question

    S^2 = Sum(Xi-X)^2/n-1

    I'm not sure what exactly Xi means. Is it the sum of a single data point minus the sample mean squared?

    Thanks
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  2. #2
    MHF Contributor matheagle's Avatar
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    the data is X_1, X_2,\ldots X_n

    You're summing i from 1 to n, and it's \bar X, not X.

    S^2={\sum_{i=1}^n (X_i-\bar X)^2\over n-1}

    ={ (X_1-\bar X)^2 + (X_2-\bar X)^2+(X_3-\bar X)^2+\cdots +(X_n-\bar X)^2 \over n-1}

    OR

    ={\sum_{i=1}^n X^2_i-n\bar X^2\over n-1}
    Last edited by matheagle; June 26th 2009 at 10:55 PM.
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