To cite ipw in publications use:

Willem M. van der Wal, Ronald B. Geskus (2011). ipw: An R Package for Inverse Probability Weighting. Journal of Statistical Software, 43(13), 1-23. URL http://www.jstatsoft.org/v43/i13/.

Corresponding BibTeX entry:

  @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},
    url = {http://www.jstatsoft.org/v43/i13/},
  }