Nondifferential misclassification tends to bias ratio measures of association in what direction?

Exercises 17.13 Smoking cessation trial. Exercise 17.4 considered a randomized trial of smoking cessation in which treatment with bupropion in combination with a nicotine patch was compared to use of a nicotine patch alone. The treatment group had success in 87 of 245 participants. The control group had successes in 40 of 244 participants. Calculate the difference in proportions and a 95%

confidence interval for p1 − p2

. Why is this confidence interval more useful than the hypothesis test results calculated in Exercise 17.4?