To cite np in publications use:

Tristen Hayfield and Jeffrey S. Racine (2008). Nonparametric Econometrics: The np Package. Journal of Statistical Software 27(5). URL http://www.jstatsoft.org/v27/i05/.

Corresponding BibTeX entry:

  @Article{,
    title = {Nonparametric Econometrics: The np Package},
    author = {Tristen Hayfield and Jeffrey S. Racine},
    journal = {Journal of Statistical Software},
    year = {2008},
    volume = {27},
    number = {5},
    url = {http://www.jstatsoft.org/v27/i05/},
  }