Closed-form confidence intervals for saved time using summary statistics in Alzheimer's disease studies.

Saved time is used in Alzheimer's disease (AD) trials as an easy interpretation of the treatment benefit to communicate with patients, family members, and caregivers. The projection approach is frequently applied to estimate saved time and its confidence interval (CI) by using the placebo or treatment disease progression curves. The estimated standard error of saved time by using these existing methods does not account for the correlation between outcomes. In addition, there was no closed-form CI for researchers to use in practice. To fill this critical gap, we derive the closed-form CI for saved time estimated from the placebo or treatment disease progression curves. We compare them with regard to coverage probability and interval width under various disease progression patterns that are commonly observed in AD symptomatic therapy and disease-modifying therapy trials. Data from the phase 3 donanemab trials are used to illustrate the application of the new CI methods.