Results 1 to 5 of 5

Math Help - AP Stats Questions

  1. #1
    Newbie
    Joined
    May 2008
    Posts
    3

    Exclamation AP Stats Questions

    I don't get how to do these two inter-related problems. I have the answer to the first one, but not the second. The questions are:

    1. A random sample of 4 Herefords, each with a frame size of three (on a one-to-seven scale), gave a sample mean weight of 452 kg. The known population standarad deviation is 12 kg. A 95% confidence interval for the average weight of all Herefords of this frame size is (using an "exact" confidence interval):

    The answer is (432.9, 471.1), but I was getting (440.2, 463.8). How do you get the right answer?

    2. Referring to the previous question, about how many animals should be sampled (in total) in order to be 95% confident of determining the true mean weight WITHIN 2 kg?

    The answer is either 138, 139, 140, 550, or 190. Keep in mind if you get a non-whole answer (which you will), you must round up, no matter what.

    Thanks for your help.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by David_is_a_LOSTaway View Post
    I don't get how to do these two inter-related problems. I have the answer to the first one, but not the second. The questions are:

    1. A random sample of 4 Herefords, each with a frame size of three (on a one-to-seven scale), gave a sample mean weight of 452 kg. The known population standarad deviation is 12 kg. A 95% confidence interval for the average weight of all Herefords of this frame size is (using an "exact" confidence interval):

    The answer is (432.9, 471.1), but I was getting (440.2, 463.8). How do you get the right answer?

    2. Referring to the previous question, about how many animals should be sampled (in total) in order to be 95% confident of determining the true mean weight WITHIN 2 kg?

    The answer is either 138, 139, 140, 550, or 190. Keep in mind if you get a non-whole answer (which you will), you must round up, no matter what.

    Thanks for your help.
    Can you show your working.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Grand Panjandrum
    Joined
    Nov 2005
    From
    someplace
    Posts
    14,972
    Thanks
    4
    Quote Originally Posted by David_is_a_LOSTaway View Post
    I don't get how to do these two inter-related problems. I have the answer to the first one, but not the second. The questions are:

    1. A random sample of 4 Herefords, each with a frame size of three (on a one-to-seven scale), gave a sample mean weight of 452 kg. The known population standarad deviation is 12 kg. A 95% confidence interval for the average weight of all Herefords of this frame size is (using an "exact" confidence interval):

    The answer is (432.9, 471.1), but I was getting (440.2, 463.8). How do you get the right answer?
    T-distribution 3 degrees of freedom and standard error of 12/sqrt(4) gives an
    interval of +/- 3.18 standard errors about the sample mean.

    RonL
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    May 2008
    Posts
    3
    Thanks for all your help, I get it now. I was using the wrong formula, one that didn't apply.

    How do I do the second part of the problem?
    2. Referring to the previous question, about how many animals should be sampled (in total) in order to be 95% confident of determining the true mean weight WITHIN 2 kg?

    The answer is either 138, 139, 140, 550, or 190. Keep in mind if you get a non-whole answer (which you will), you must round up, no matter what.
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by David_is_a_LOSTaway View Post
    Thanks for all your help, I get it now. I was using the wrong formula, one that didn't apply.

    How do I do the second part of the problem?
    Quote:
    2. Referring to the previous question, about how many animals should be sampled (in total) in order to be 95% confident of determining the true mean weight WITHIN 2 kg?

    The answer is either 138, 139, 140, 550, or 190. Keep in mind if you get a non-whole answer (which you will), you must round up, no matter what.
    You want the value of n such that the 95% confidence interval is 452 \pm 2.

    Therefore  2 = z_{\alpha/2} \frac{\sigma}{\sqrt{n}} = z_{0.025} \frac{12}{\sqrt{n}} . You look up z_{0.025} in a table of critical values. Substitute. Solve for n.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Stats exam questions I need help with
    Posted in the Advanced Statistics Forum
    Replies: 2
    Last Post: August 25th 2011, 05:18 AM
  2. 2 basic stats questions
    Posted in the Statistics Forum
    Replies: 1
    Last Post: November 22nd 2009, 03:25 PM
  3. simple stats questions #2
    Posted in the Statistics Forum
    Replies: 1
    Last Post: September 5th 2009, 04:49 AM
  4. simple stats questions
    Posted in the Statistics Forum
    Replies: 2
    Last Post: September 3rd 2009, 06:07 AM
  5. questions about stats help
    Posted in the Advanced Statistics Forum
    Replies: 0
    Last Post: July 11th 2007, 12:00 PM

Search Tags


/mathhelpforum @mathhelpforum