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Math Help - Standard Deviation problem/help

  1. #1
    Newbie
    Joined
    Jan 2010
    Posts
    1

    Angry Standard Deviation problem/help

    Hi all,

    I need to work out the standard dev for the following table

    Crushing Strength N/mm2
    25
    28
    31
    34
    37
    40
    Frequency (f)
    15
    30
    42
    36
    24
    18

    However I have used a spreadsheet to calculate the answer and I'm coming up with 10.46422477. This is also the figure I get when working it out on the calculator.

    The problem is the mean I worked manually using formula: (sigma) crushing strength x frequency / (sigma) frequency, so 5349/165 comes to 32.41818182. But the mean using excel comes to 27.5.

    Can someone work out the standard deviation for me so I can see where I'm going wrong?

    Thanks ppl
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  2. #2
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by paramed View Post
    Hi all,

    I need to work out the standard dev for the following table

    Crushing Strength N/mm2
    25
    28
    31
    34
    37
    40
    Frequency (f)
    15
    30
    42
    36
    24
    18

    However I have used a spreadsheet to calculate the answer and I'm coming up with 10.46422477. This is also the figure I get when working it out on the calculator.

    The problem is the mean I worked manually using formula: (sigma) crushing strength x frequency / (sigma) frequency, so 5349/165 comes to 32.41818182. But the mean using excel comes to 27.5.

    Can someone work out the standard deviation for me so I can see where I'm going wrong?

    Thanks ppl
    Check the Excel calculation for the mean. Also check your calculation of the SD, it is implausible that all the data is within less than 1 SD of the mean.

    CB
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