To cite the urn package in publications use:

Altman, M., (2007) (Software version: R Package, Urn, version 2.1)

Corresponding BibTeX entry:

  @Unpublished{,
    title = {URN: An R-Package for Resampling without Replaacement},
    author = {M. Altman},
    year = {2007},
    note = {Software: R Package, urn , version 2.1 },
    url = {http://www.hmdc.harvard.edu/micah_altman/},
  }