54. In medical investigations, the ratio up1/p2 is often of more interest than the difference p1  p2 (e.g., individuals given treatment 1 are how many times as likely to recover as those given treatment 2?). Let

 / . When m and n are both large, the statistic ln( ) has approximately a normal distribution with approximate mean value ln(u) and approximate standard deviation [(m  x)/(mx)  (n  y)/(ny)]1/2.

a. Use these facts to obtain a large-sample 95% CI formula for estimating ln(u), and then a CI for u itself.

b. Return to the heart attack data of Example 1.3, and calculate an interval of plausible values for u at the 95% con dence level. What does this interval suggest about the ef cacy of the aspirin treatment?