Please cite the nph package by the following reference (open access):

Ristl R, Ballarini N, Götte H, Schüler A, Posch M, König F (2021). “Delayed treatment effects, treatment switching and heterogeneous patient populations: How to design and analyze RCTs in oncology.” Pharmaceutical statistics, 20(1), 129–145.

Corresponding BibTeX entry:

  @Article{,
    title = {Delayed treatment effects, treatment switching and
      heterogeneous patient populations: How to design and analyze RCTs
      in oncology},
    author = {Robin Ristl and Nicolas Ballarini and Heiko G{\"o}tte and
      Armin Sch{\"u}ler and Martin Posch and Franz K{\"o}nig},
    journal = {Pharmaceutical statistics},
    volume = {20},
    number = {1},
    pages = {129--145},
    year = {2021},
  }