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Variance problem

  1. Apr 2, 2007 #1
    Hi all:
    If I have a dataset V of the size n*1. Assume that the mean of the dataset is 0 and var(V)=x is its variance. If I want to modify this dataset so that the variance of the new dataset will be var(V_hat)=y. The errors are spreaded average on each element in the dataset. What I did is first calculate the error which need to be speaded on each elements

    v_hat - v = delta = sqrt[(y-x)*(n-1) / n] (0)


    var(V) = x = sum(v^2)/(n-1) (1)
    var(V_hat) = y = sum(v_hat^2)/(n-1) (2)

    (2)-(1) and rearrange I got the equation (0);

    However, this didn't give me the supposed answer. Could anyone point out what's the error in this or if there is any better methods please?
    Thanks a lot!!!
  2. jcsd
  3. Apr 2, 2007 #2


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    Science Advisor

    I am not sure what limits you have on the modification. However, you can multiply each element of V by sqrt(y/x) to get what you want.
  4. Apr 2, 2007 #3
    What a nice and easy way. How stupid I am. Thanks a lot!!!
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