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

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Armitage, Peter, CK McPherson, and BC Rowe. 1969. “Repeated Significance Tests on Accumulating Data.” Journal of the Royal Statistical Society: Series A (General) 132 (2): 235–44.
Biologics Evaluation, Center for, and Research (CBER). 2018. “Guidance for Industry. Adaptive Designs for Clinical Trials of Drugs and Biologics.”
Chan, Ivan SF, and Norman R Bohidar. 1998. “Exact Power and Sample Size for Vaccine Efficacy Studies.” Communications in Statistics - Theory and Methods 27 (6): 1305–22.
Chen, YH Joshua, David L DeMets, and KK Gordon Lan. 2004. “Increasing the Sample Size When the Unblinded Interim Result Is Promising.” Statistics in Medicine 23 (7): 1023–38.
DeMets, David L, Curt D Furberg, and Lawrence M Friedman, eds. 2006. Data Monitoring in Clinical Trials: A Case Studies Approach. Springer-Verlag New York.
Farrington, Conor P, and Godfrey Manning. 1990. “Test Statistics and Sample Size Formulae for Comparative Binomial Trials with Null Hypothesis of Non-Zero Risk Difference or Non-Unity Relative Risk.” Statistics in Medicine 9 (12): 1447–54.
Fleiss, Joseph L, Alex Tytun, and Hans K Ury. 1980. “A Simple Approximation for Calculating Sample Sizes for Comparing Independent Proportions.” Biometrics, 343–46.
Gordon, Ian, and Ray Watson. 1996. “The Myth of Continuity-Corrected Sample Size Formulae.” Biometrics, 71–76.
Gordon Lan, KK, and David L DeMets. 1983. “Discrete Sequential Boundaries for Clinical Trials.” Biometrika 70 (3): 659–63.
Jennison, Christopher, and Bruce W Turnbull. 1999. Group Sequential Methods with Applications to Clinical Trials. CRC Press.
Kim, Kyungmann, and Anastasios A Tsiatis. 1990. “Study Duration for Clinical Trials with Survival Response and Early Stopping Rule.” Biometrics, 81–92.
Lachin, John M. 1981. “Introduction to Sample Size Determination and Power Analysis for Clinical Trials.” Controlled Clinical Trials 2 (2): 93–113.
Lachin, John M, and Mary A Foulkes. 1986. “Evaluation of Sample Size and Power for Analyses of Survival with Allowance for Nonuniform Patient Entry, Losses to Follow-up, Noncompliance, and Stratification.” Biometrics, 507–19.
Miettinen, Olli, and Markku Nurminen. 1985. “Comparative Analysis of Two Rates.” Statistics in Medicine 4 (2): 213–26.
O’Brien, Peter C, and Thomas R Fleming. 1979. “A Multiple Testing Procedure for Clinical Trials.” Biometrics, 549–56.
Pocock, Stuart J. 1977. “Group Sequential Methods in the Design and Analysis of Clinical Trials.” Biometrika 64 (2): 191–99.
Polack, Fernando P, Stephen J Thomas, Nicholas Kitchin, Judith Absalon, Alejandra Gurtman, Stephen Lockhart, John L Perez, et al. 2020. “Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine.” New England Journal of Medicine 383 (27): 2603–15.
Proschan, Michael A, KK Gordon Lan, and Janet Turk Wittes. 2006. Statistical Monitoring of Clinical Trials: A Unified Approach. Springer Science & Business Media.
Snedecor, George W, and William G Cochran. 1989. Statistical Methods. 8th ed. Ames, IA: Iowa State University Press.
Whitehead, John. 1992. “Overrunning and Underrunning in Sequential Clinical Trials.” Controlled Clinical Trials 13 (2): 106–21.
Zhou, Jing, Adeniyi Adewale, Yue Shentu, Jiajun Liu, and Keaven Anderson. 2014. “Information-Based Sample Size Re-Estimation in Group Sequential Design for Longitudinal Trials.” Statistics in Medicine 33 (22): 3801–14.