To cite ipw in publications use:

  van der Wal WM, Geskus RB (2011). “ipw: An R Package for Inverse
  Probability Weighting.” _Journal of Statistical Software_, *43*(13),
  1-23. doi:10.18637/jss.v043.i13
  <https://doi.org/10.18637/jss.v043.i13>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{ipw}: An {R} Package for Inverse Probability Weighting},
    author = {Willem M. {van der Wal} and Ronald B. Geskus},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {43},
    number = {13},
    pages = {1--23},
    doi = {10.18637/jss.v043.i13},
  }