Conover, W.J. (1999) – Practical Nonparametric Statistics. John Wiley& Sons. New York.Murteira, B. et al (2002) – Introdução à Estatística, McGraw Hill. De Portugal.Rohatgi, M., and Wolfe, D.A. (1999) - Nonparametric Statistical Methods. New York: John Wiley & Sons. Siegel, S., and Castellan, N.Y. (1988) - Nonparametric Statistics for the Behaviour Sciences, 2nd ed. New York: McGraw Hill.Sprent, P. (1993) - Applied Nonparametric Statistical Models, 2nd ed. London: Chapman & Hall: