Group sequential designs using a family of type I error probability spending functions.

Performing planned or unplanned interim analyses on accumulating data in clinical trials is a frequent practice. In this paper, we propose a general one-parameter family of type I error probability spending functions to construct customized group sequential boundaries with unequal increments in information time. This proposed family generalized the spending functions of Lan and DeMets1 and Kim and DeMets.2 We give an example to illustrate the use of this family.