It is very expensive to purchase software that performs group sequential monitoring of clinical trials (sometimes called interim analysis). Group sequential monitoring is looking at a trial at selected time points during the study to see if you should stop the study early. There are a couple of functions in R that will do simple calculations, and the price, of course, is free.
The HMisc library created by Frank Harrell has a function ldBands that uses the Lan-DeMets method with flexible alpha spending functions.
The seqmon library created by David A. Schoenfeld has a seqmon function that uses the algorithm by Amitage, McPherson and Rowe. An example they cite shows boundaries using the O’Brien Fleming approach.
I will try to document some of the details of these algorithms and test them against some well known examples when I have time.
Further reading: