The files on this website accompany a paper about how to perform sequential analyses in order to perform high-powered studies efficiently, which is available here:
Lakens, D. (2014). Performing high-powered studies efficiently with sequential analyses: Sequential analyses. European Journal of Social Psychology, 44(7), 701–710. https://doi.org/10.1002/ejsp.2023
The step by step guides explain how to perform sequential analyses in R or using the dedicated siftware WinDL. The spreadsheet facilitates performing some necessary calculations.
The WinDL software is now available from: https://www.biostat.wisc.edu/content/lan-demets-method-statistical-programs-clinical-trials