6.17 The state of stress in the United States. Since 2007, the American Psychological Association has supported an annual nationwide survey to examine stress across the United States.7 This year, a total of 720 millennials (18- to 33-year-olds) were asked to indicate their average stress level (on a 10-point scale) during the past month. The mean score was 5.5. Assume that the population standard deviation is 2.8.

  1. (a) Give the margin of error and find the 95% confidence interval for this sample.

  2. (b) Repeat these calculations for a 99% confidence interval. How do the results compare with those in part (a)?