To cite package ‘NBDesign’ in publications use:

  Luo X (2020). _NBDesign: Design and Monitoring of Clinical Trials
  with Negative Binomial Endpoint_. R package version 2.0.0,
  <https://CRAN.R-project.org/package=NBDesign>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {NBDesign: Design and Monitoring of Clinical Trials with Negative Binomial
Endpoint},
    author = {Xiaodong Luo},
    year = {2020},
    note = {R package version 2.0.0},
    url = {https://CRAN.R-project.org/package=NBDesign},
  }