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Math Help - Finding the combined standard deviation

  1. #1
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    Nov 2012
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    singapore
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    Finding the combined standard deviation

    Suppose i have 2 sets


    Set A: number of items = 10, mean = 2.4 , S.D = 0.8

    Set B: number of items = 5, mean = 2, S.D = 1.2


    Ok i can find the combined mean easily, but how am i supposed to find the combined standard deviation?


    Any help would be greatly appreciated...
    Thanks
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  2. #2
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    Apr 2013
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    Malaysia
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    Re: Finding the combined standard deviation

    1. Var(A+B) = Var(A) + Var(B) ( Given that both observations are independent; Otherwise, Var(A+B) =/= Var(A) + Var(B) )
    2. 1.2 square + 0.8 square = 2.08 (this is the combined variance)
    3. square root 2.08 = 1.442 (combined standard deviation)
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