17. A study was done on 41 rst-year medical students to see if their anxiety levels changed during the rst semester. One measure used was the level of serum cortisol, which is associated with stress.

For each of the 41 students the level was compared during nals at the end of the semester against the level in the rst week of classes. The average difference was 2.08 with a standard deviation of 7.88.

Find a 95% lower con dence bound for the population mean difference m. Does the bound suggest that the mean population stress change is necessarily positive?