ZEILEIS, A.; KÖLL, S.; GRAHAM, N. Various Versatile Variances: An Object-Oriented Implementation of Clustered Covariances in R. Journal of Statistical Software, [S. l.], v. 95, n. 1, p. 1–36, 2020. DOI: 10.18637/jss.v095.i01. Disponível em: https://www.jstatsoft.org/index.php/jss/article/view/v095i01. Acesso em: 8 feb. 2023.