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# Statistics: Solving for the Confidence Interval

Question: 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 2603564035360 268369428536 268396469536162338403536536130

a) Construct a 95% 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% confidence?
d) If this is not a reasonable requirement, suggest one that is.

#### Solution Preview

Sample mean = x-bar = 346.5
The sample standard deviation is s = 170.38
The t-value for the degrees of freedom (dof) = 20 - 1 = 19 on a two-tailed test with alpha = 0.05 is 2.093
Margin of ...

#### Solution Summary

This solution is comprised of a 135 word response, which is complete, neat and shows a step-by-step methodology for all of the required calculations for the statistics problems in question.

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