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Math Help - Confidence Interval??

  1. #1
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    Question Confidence Interval??

    A sample of 20 pages was taken without replacement from the 1,591-page phone directory
    Ameritech Pages Plus Yellow Pages. On each page, the mean area devoted to display ads was measured (a display ad is a large block of multicolored illustrations, maps, and text). The data (in
    square millimeters) are shown below:

    0 260 356 403 536 0 268 369 428 536
    268 396 469 536 162 338 403 536 536 130

    (a) Construct a 95 percent confidence interval for the true mean.
    (b) Why might normality be an issue here?
    (c) What sample size would be needed to obtain an error of 10 square millimeters with 99 percent confidence?
    (d) If this is not a reasonable requirement, suggest one that is.



    Ok, I am stuck, I have this:

    X M (X-M) (X-M)2
    0 346.5 -346.5 120062.25
    260 346.5 86.5 7482.25
    356 346.5 9.5 90.25
    403 346.5 56.5 3192.25
    536 346.5 189.5 35910.25
    0 346.5 -346.5 120062.25
    268 346.5 -78.5 6162.25
    369 346.5 22.5 506.25
    428 346.5 81.5 6642.25
    536 346.5 189.5 35910.25
    268 346.5 -78.5 6162.25
    396 346.5 49.5 2450.25
    469 346.5 122.5 15005.25
    536 346.5 189.5 35910.25
    162 346.5 -184.5 34040.25
    338 346.5 -8.5 72.25
    403 346.5 56.5 3192.25
    536 346.5 189.5 35910.25
    536 346.5 189.5 35910.25
    130 346.5 -216.5 46972.25


    where do I go from here?? Or better yet, am I on the right path?
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  2. #2
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    Quote Originally Posted by foofergutierrez View Post
    A sample of 20 pages was taken without replacement from the 1,591-page phone directory
    Ameritech Pages Plus Yellow Pages. On each page, the mean area devoted to display ads was measured (a display ad is a large block of multicolored illustrations, maps, and text). The data (in
    square millimeters) are shown below:

    0 260 356 403 536 0 268 369 428 536
    268 396 469 536 162 338 403 536 536 130

    (a) Construct a 95 percent confidence interval for the true mean.
    (b) Why might normality be an issue here?
    (c) What sample size would be needed to obtain an error of 10 square millimeters with 99 percent confidence?
    (d) If this is not a reasonable requirement, suggest one that is.



    Ok, I am stuck, I have this:

    X M (X-M) (X-M)2
    0 346.5 -346.5 120062.25
    260 346.5 86.5 7482.25
    356 346.5 9.5 90.25
    403 346.5 56.5 3192.25
    536 346.5 189.5 35910.25
    0 346.5 -346.5 120062.25
    268 346.5 -78.5 6162.25
    369 346.5 22.5 506.25
    428 346.5 81.5 6642.25
    536 346.5 189.5 35910.25
    268 346.5 -78.5 6162.25
    396 346.5 49.5 2450.25
    469 346.5 122.5 15005.25
    536 346.5 189.5 35910.25
    162 346.5 -184.5 34040.25
    338 346.5 -8.5 72.25
    403 346.5 56.5 3192.25
    536 346.5 189.5 35910.25
    536 346.5 189.5 35910.25
    130 346.5 -216.5 46972.25


    where do I go from here?? Or better yet, am I on the right path?
    Same assignment every year I see. Read this thread: http://www.mathhelpforum.com/math-he...tion-help.html
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  3. #3
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    I am now more confused than ever but thanks for your help.
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  4. #4
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    Does this look more like it?


    count 20
    mean 346.50
    sample variance 29,028.79
    sample standard deviation 170.38

    confidence interval 99.% lower 237.50
    confidence interval 99.% upper 455.50
    half-width 109.00

    empirical rule
    mean - 1s 176.12
    mean + 1s 516.88
    percent in interval (68.26%) 55.0%
    mean - 2s 5.74
    mean + 2s 687.26
    percent in interval (95.44%) 90.0%
    mean - 3s -164.64
    mean + 3s 857.64
    percent in interval (99.73%) 100.0%
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  5. #5
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    Quote Originally Posted by foofergutierrez View Post
    [snip]

    count 20
    mean 346.50
    sample variance 29,028.79
    sample standard deviation 170.38

    Mr F says: This is all OK.

    confidence interval 99.% lower 237.50 Mr F says: *Ahem* I quote: "Construct a 95 percent confidence interval for the true mean." I get 264.7.

    confidence interval 99.% upper 455.50 Mr F says: Ditto the quote.
    [snip]
    ..
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  6. #6
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    Cool confidence interval 99.%

    I have obtained this information on excel
    count 20
    mean 346.50
    sample variance 29,028.79
    sample standard deviation 170.38

    but, I do not understand how to get the CI by using excel

    0 260 356 403 536 0 268 369 428 536
    268 396 469 536 162 338 403 536 536 130

    (a) Construct a 95 percent confidence interval for the true mean.

    Thanks - Wjames23
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  7. #7
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    Quote Originally Posted by wjames23 View Post
    I have obtained this information on excel
    count 20
    mean 346.50
    sample variance 29,028.79
    sample standard deviation 170.38

    but, I do not understand how to get the CI by using excel

    0 260 356 403 536 0 268 369 428 536
    268 396 469 536 162 338 403 536 536 130

    (a) Construct a 95 percent confidence interval for the true mean.

    Thanks - Wjames23
    Perhaps you're meant to construct the interval by hand using the appropriate formulae.
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