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Math Help - standard deviation of two data sets(combined)

  1. #1
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    standard deviation of two data sets(combined)

    Hi;
    I have two sets of data and need to find there combined standard deviation.

    What I know 1: for a set of 10 numbers sum of x=50 and sum of x(squared)=310.

    1: for a set of 15 numbers sum of x=86 and sum of x(squared)=568.


    I know how to find the standard deviation ofthe individual sets but don't know how to
    combine them.

    Thanks.
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  2. #2
    Senior Member Sambit's Avatar
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    Re: standard deviation of two data sets(combined)

    I am giving the formula for the Combined Variance. Once you find this, the combined standard deviation is only a single step away...
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  3. #3
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    Re: standard deviation of two data sets(combined)

    Yes thank you I have seen this online but I'm getting a different answer
    from it than the one in my book which says 3.25.

    Can someone help me out with this?
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  4. #4
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    Re: standard deviation of two data sets(combined)

    I now now why me book says 3.25 they have just added the two standard deviations
    which I can kind of see why this is wrong.

    Can someone please give me the right answer so that I know for myself,
    from the formula provided by Sambit I get a combined standard deviation of 5.5876 is this correct?

    Please pop over to the calculator forum and help me out with putting two groups of frequency data in
    and getting the combined standard deviation out.

    Thank you.
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