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Math Help - Simple stats problem : Unable to get final answer

  1. #1
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    Simple stats problem : Unable to get final answer

    I have a simple stats problem where the following table is given.

    Speed (Km/h)--------Frequency-----------Cumulative Frequency

    0<s<=40--------------14----------------------14
    40<s<=60-------------51----------------------65
    60<s<=70-------------157---------------------222
    70<s<=80-------------141---------------------363
    80<s<=100------------37----------------------400

    I am asked to find the median which i can find easily 51, but the standard deviation i tried all sorts of values but unable to get the given answer which is 64.6. I am getting like 13.67. Can someone guide me out on this issue ?
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by tester85 View Post
    I have a simple stats problem where the following table is given.

    Speed (Km/h)--------Frequency-----------Cumulative Frequency

    0<s<=40--------------14----------------------14
    40<s<=60-------------51----------------------65
    60<s<=70-------------157---------------------222
    70<s<=80-------------141---------------------363
    80<s<=100------------37----------------------400

    I am asked to find the median which i can find easily 51, but the standard deviation i tried all sorts of values but unable to get the given answer which is 64.6. I am getting like 13.67. Can someone guide me out on this issue ?
    The standard deviation cannot be 64.6.

    The mean is:

    m=\frac{\sum_{i=1}^5 f_ix_i} {\sum_{i=1}^5 f_i}

    where x_i is the cell mark and f_i is the frequency of the i th cell respectivly.

    then:

    s^2=\frac{\sum_{i=1}^5 f_i(x_i-m)^2}{\sum_{i=1}^5 f_i}

    is the variance and s is the standard deviation.


    CB
    Last edited by CaptainBlack; October 26th 2008 at 10:44 PM.
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  3. #3
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    I use a one of the standard deviation formula.

    \sum xifi = 26940 \sum xi^2 fi = 1943530

    s^2 = 1/400[ 1943530 - (27340)^2 /400 ] = 13.67

    According to the formula stated i got 67.35 for the mean is that correct ?

    May i know what is the value you got for the mean and standard deviation?
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