We provide a brief set of references that may be of interest to the reader.

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Proschan, Michael A, KK Gordon Lan, and Janet Turk Wittes. Statistical Monitoring of Clinical Trials: A Unified Approach. Springer Science & Business Media, 2006.
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Zhou, Jing, Adeniyi Adewale, Yue Shentu, Jiajun Liu, and Keaven Anderson. “Information-Based Sample Size Re-Estimation in Group Sequential Design for Longitudinal Trials.” Statistics in Medicine 33, no. 22 (2014): 3801–14.