To cite np in publications use:

Hayfield T, Racine JS (2008). “Nonparametric Econometrics: The np Package.” Journal of Statistical Software, 27(5), 1-32. doi:10.18637/jss.v027.i05, https://www.jstatsoft.org/index.php/jss/article/view/v027i05.

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 =
      {https://www.jstatsoft.org/index.php/jss/article/view/v027i05},
    doi = {10.18637/jss.v027.i05},
    pages = {1-32},
  }