Thread: 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?

Originally Posted by foofergutierrez

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

I am now more confused than ever but thanks for your help.

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%

Originally Posted by foofergutierrez

[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]

..

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

Originally Posted by wjames23

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.