The S4 sequential design class
The sequential design class stores the information of a sequential design, including revision history.
an object of the class "sequential.design"
David A. Schoendfeld, PhD and Hui Zheng, PhD
Proschan, MA; Lan, KKG; Wittes JT,"Statistical Monitoring of Clinical Trials: A Unified Approach", Chapter 6, Springer 2006.
Schoenfeld DA, "A Simple Algorithm for Designing Group Sequential Clinical Trials", Biometrics. 2001 Sep;57(3):972-4.