Results 1 to 2 of 2

Thread: Standard Normal Table

  1. #1
    Member
    Joined
    Apr 2013
    From
    Mahinog
    Posts
    98
    Thanks
    5

    Standard Normal Table

    Given X~N(, 1), find the sample size required to ensure that the probability thatx is within 0.1 of is greater than 0.95.

    Depending on the table that I used, I got 2 different answers.
    i) P(-0.1<
    x - <0.1) >0.95
    2P(Z<0.1/(1/sqrt(n))) -1>0.95
    P
    (Z<0.1/(1/sqrt(n))) >0.975
    0.1/(1/sqrt(n))>1.96
    n>384.16
    n = 385

    ii)
    P(-0.1<x - <0.1) >0.95
    1-2P(Z>0.1/(1/sqrt(n)))>0.95
    P(Z>0.1/(1/sqrt(n)))<0.025
    0.1/(1/sqrt(n))<1.96
    n<384.16
    n = 383

    I don't understand, where did I do wrong?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Junior Member
    Joined
    Aug 2017
    From
    Finland
    Posts
    34
    Thanks
    10

    Re: Standard Normal Table

    The issue is in the method ii).
    You have used $P(Z>.01\sqrt{n})<0.025 \implies 0.1\sqrt{n}<1.96$ which is wrong. It should be $0.1\sqrt{n}>1.96$.
    Note that you are looking for upper tail of the distribution in $P(Z>.01\sqrt{n})<0.025$.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 9
    Last Post: May 20th 2011, 09:29 PM
  2. Normal Distribution table
    Posted in the Advanced Statistics Forum
    Replies: 2
    Last Post: May 6th 2010, 10:22 PM
  3. Frequency Table Standard Deviation
    Posted in the Advanced Statistics Forum
    Replies: 9
    Last Post: Dec 12th 2009, 05:00 AM
  4. Replies: 5
    Last Post: Jul 26th 2008, 01:53 PM
  5. Normal Distribution/Z-table usage
    Posted in the Advanced Statistics Forum
    Replies: 4
    Last Post: Jun 9th 2008, 11:28 PM

/mathhelpforum @mathhelpforum