Confidence interval for true mean and determination of sample size.
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 (insquare millimeters) are shown below:
0 260 356 403 536 0 268 369 428 536 268 396 469 536 162 338 403 536 130
(a) Construct a 95 percent confidence interval for the true mean.
(b) Why may normality be an issue here?
(c) What sample size would be necessary to get an error of ±10 square millimeters with 99 percent confidence?
(d) If this is not a reasonable requirement, suggest one that is.