KHOO, Michael B.C.; SIM, J.N. Enhancements for the $S^2$ Control Chart. MATEMATIKA, [S. l.], v. 21, p. 133–141, 2005. DOI: 10.11113/matematika.v21.n.517. Disponível em: https://matematika.utm.my/index.php/matematika/article/view/517. Acesso em: 4 may. 2024.