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Help finding percentage of data

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  • #42552

    Mario
    Participant

    if i given the problem to find the percentage of data that falls between plus/minus 2.3 standard deviation from the mean, how do i determine this?  I look at z chart and see low numbers but i beleive answer should be something around 95%.
    am i not using correctly?

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    #134406

    Mr IAM
    Participant

    A typical Z table is giving you the % beyond the z-value not the % between the mean and the Z value.
    So.. if you look up a Z-value of 2.3 you get .01072 or 1.07% right? So, you need to subtract this value from 1 to the percentage under the Z-value. 1-.01072 = .9892.
    So.. if you have 1.07% on one side, what percentage would you have on the other side?  What is left?

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    #134409

    Mario
    Participant

    thank you for your answer but i am still confused.  if i look at 2.3 on the z table, i get .04893.  i feel i am looking at wrong table?  i search this site and found one that looks just like mine with same answer.

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    #134410

    Mr IAM
    Participant

    No.. it is .4893 which is a lot different then .04893.  That is a different Z-table which is giving you the % between the mean and the Z-Value.
    So, all you need to do is double the number. .4893*2 = .9786 or 97.86% is between +/- 2.3.
    You would get the same answer using my other post right?  1.07*2 = 2.14.  100-2.14 = 97.86!
    Magic!
    I hope that clears it up.  The differnce was the Z-table.  Most Z-tables in stats books will not be like the one you found on this site.
    Cheers!

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    #134573

    Sathiya
    Member

    Just now i have gone through your doubt.
    To find area under the curve, it is enough to use normdist funtion in excel.
    The function is normdist(X, mean, stdev, cumulative).
    In your case it is normdist(2.3, 0,1,1) = 0.9892, which is 98%. So 98% of your data falls within +/-2.3 stdev.
    regards

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