To cite package 'plink' in publications use:

Jonathan P. Weeks (2009). plink: IRT separate calibration linking methods. (R package). URL http://cran.r-project.org/web/packages/plink/index.html

Corresponding BibTeX entry:

  @Manual{,
    title = {plink: {IRT} separate calibration linking methods},
    author = {Jonathan P. Weeks},
    year = {2009},
    note = {R package},
    url = {http://cran.r-project.org/web/packages/plink/index.html},
  }