To cite package ‘futility’ in publications use:

  Zhuang Y, Juraska M (2019). _futility: Interim Analysis of
  Operational Futility in Randomized Trials with Time-to-Event
  Endpoints and Fixed Follow-Up_. R package version 0.4,
  <https://github.com/mjuraska/futility>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {futility: Interim Analysis of Operational Futility in Randomized Trials
with Time-to-Event Endpoints and Fixed Follow-Up},
    author = {Yingying Zhuang and Michal Juraska},
    year = {2019},
    note = {R package version 0.4},
    url = {https://github.com/mjuraska/futility},
  }