The probability of rejection is given by the formula q = d/[n-(w/2)]. The numerator is the number of rejections during the interval - that's the easy part. In the denominator, we have to account for the patients lost to follow-up, so we subtract the number lost to follow-up. We divide by 2 because we don't know when in the interval they were lost, and therefore assume they were lost half-way through.