Currently, the design of group sequential clinical trials requires choosing
among several distinct design categories, design scales, and strategies fo
r determining stopping rules. This approach can limit the design selection
process so that clinical issues are not fully addressed. This paper describ
es a family of designs that unifies previous approaches and allows continuo
us movement among the previous categories. This unified approach facilitate
s the process of tailoring the design to address important clinical issues.
The unified family of designs is constructed from a generalization of a fo
ur-boundary group sequential design in which the shape and location of each
boundary can bt: independently specified. Methods for implementing the des
ign using error-spending functions are described. Examples illustrating the
use of the design family are also presented.