In clinical trials of new medical treatments, it is standard practice to monitor accumulating data with specialized statistical methods called group sequential methods. This work surveys and extends current methods for planning and conducting interim analyses, and describes group sequential stopping rules which can reduce average study length while controlling error probabilities. Procedures are presented in a manner which allows their easy implementation to a variety of data types arising from clinical trials. Of interest to medical statisticians in the pharmaceutical industry and medical research, as well as academic statisticians. The authors are professors of statistics at the University of Bath, UK, and at Cornell University. Annotation c. Book News, Inc., Portland, OR (booknews.com)