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Math Help - I need some help

  1. #1
    Newbie
    Joined
    Mar 2006
    Posts
    14

    I need some help

    I am having trouble getting the mean and standeard deviation. could you please show me how ? thanks

    perscription data
    0.4-1.1
    1.2-1.9
    2.0-2.7
    2,8-3.5
    3.6-4.3
    Frequency
    2
    2
    4
    3
    9
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  2. #2
    Super Member
    Joined
    Oct 2007
    From
    London / Cambridge
    Posts
    591
    whats wrong?

    the mean is the sum of the mid points of each class times the class frequency divided by the total frequency

    and then the standard deviation is  \sqrt { \frac { \sum{fx^2}}{\sum{f}} - \mu^2 }

    its just a simple formula where are you having trouble ?
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  3. #3
    Newbie
    Joined
    Mar 2006
    Posts
    14
    how do i get the class mid points ?
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  4. #4
    Super Member
    Joined
    Oct 2007
    From
    London / Cambridge
    Posts
    591
    take the mean of the upper and lower boundaries.

    for example \frac{0.4 + 1.1}{2}
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