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Math Help - Statistics: Standard Normal Distribution-Confused

  1. #1
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    Smile Statistics: Standard Normal Distribution-Confused

    I know that the 1 standard deviation is 68% and a 2 standard is 95 %. What should I do first, in order too set up the problem? I came up with .975, but only from using an calulator. Am i suppose to subtract the numbers or add in the z from the z score table?

    Between 1 standard deviation below the mean and 2 standard deviation above the mean.




    More than 2 standard deviation away from the mean.

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  2. #2
    Eater of Worlds
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    Quote Originally Posted by Lydia08 View Post
    I know that the 1 standard deviation is 68% and a 2 standard is 95 %. What should I do first, in order too set up the problem? I came up with .975, but only from using an calulator. Am i suppose to subtract the numbers or add in the z from the z score table?



    Between 1 standard deviation below the mean and 2 standard deviation above the mean.







    More than 2 standard deviation away from the mean.
    1 SD below accounts for 34% (one half 68) and 2 above account for 34+13.5=47.5%

    That is because 95-68=27. 27/2=13.5

    You can see from the graph of the normal distribution.

    Add them up. one below to two above would be 34+34+13.5=81.5

    From the tables, look up z=-1 and z=2. You should get close to 81.5

    z=-1 corresponds to .1587. Then, .5-.1587=.3413

    z=2 corresponds to .9772. Then, .9772-.5=.4772

    .4772+.3413=.8185.

    See?. Close to the graph method.
    Attached Thumbnails Attached Thumbnails Statistics: Standard Normal Distribution-Confused-normal_34.gif  
    Last edited by mr fantastic; January 24th 2009 at 02:16 PM. Reason: Added quote. And flagging this post as having been moved from a duplicate thread.
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  3. #3
    Senior Member vincisonfire's Avatar
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    There are 15.9% one standard deviation below the mean and 2.2% 2 standard deviation above the mean.
    100% - 15.9% - 2.2% = ?
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