HAYFIELD, T.; RACINE, J. S. Nonparametric Econometrics: The np Package. Journal of Statistical Software, [S. l.], v. 27, n. 5, p. 1–32, 2008. DOI: 10.18637/jss.v027.i05. Disponível em: https://www.jstatsoft.org/index.php/jss/article/view/v027i05. Acesso em: 7 oct. 2022.