To cite the ordinal-package in publications use:

Christensen, R. H. B. (2015). ordinal - Regression Models for Ordinal Data. R package version 2015.6-28. http://www.cran.r-project.org/package=ordinal/.

Corresponding BibTeX entry:

  @Misc{,
    title = {ordinal---Regression Models for Ordinal Data },
    author = {R. H. B. Christensen},
    year = {2015},
    note = {R package version 2015.6-28.
      http://www.cran.r-project.org/package=ordinal/},
  }