EXAMPLE 6.12 How the Confidence Level Affects the Confidence Interval

Suppose that for the student credit card data in Example 6.10 (pages 307–308), we wanted 99% confidence. Table D tells us that for 99% confidence, . The margin of error for 99% confidence based on 532 observations is

and the 99% confidence interval is

Requiring 99%, rather than 95%, confidence has increased the margin of error from 96 to 126. Figure 6.11 compares the two intervals.