Zonta, Élio P. and Silva, J.G.C. da 1999. A practical algorithm to determine the variance of the estimator of a contrast of parameters. Pesquisa Agropecuaria Brasileira. 34, 7 (Jul. 1999), 1125–1132. DOI:https://doi.org/10.1590/S1678-3921.pab1999.v34.5315.