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Para verificar se a pequena influência da heterogeneidade das variâncias, observada em trabalho anterior, sobre os testes de Bonferroni, "t" e Tukey se devia ao fato de ter sido utilizado o erro médio que corresponde ao quadrado médio da interação tratamentos x blocos (Q.M. trat x bloco), os autores estudaram o comportamento do teste "t" com o uso de três tipos de erro: Q.M. trat x bloco, erro do contraste específico e erro ponderado dos contrastes. Foram simulados 200 experimentos, em grupos de 50, para cada uma das 56 combinações de quatro coeficientes de variação, dois efeitos de tratamentos e sete grupos de heterogeneidade, totalizando 11.200 experimentos. Foi feita a análise da variância de cada ensaio e estudados os seguintes contrastes: m1 - m3 e m2 - m3, onde m3 representa a testemunha. "A priori" foi estabelecido que o tratamento 1 seria superior à testemunha nas quantidades de 8 e 17% (para os efeitos 1 e 2 respectivamente) e o tratamento 2, nas quantidades de 5 e 12% (para os mesmos efeitos). Assim, foi empregado o teste t, unilateral, para comparação das médias nos dois contrastes. Na presente pesquisa, são válidas as seguintes conclusões: as diferenças nas porcentagens de rejeição com os três tipos de erro praticamente inexistem; as relações de variâncias com valores maiores que 6 podem ser consideradas heterogêneas, quando utilizado o erro do contraste; a porcentagem de rejeição obtida é indiretamente proporcional ao coeficiente de variação e diretamente proporcional à magnitude dos contrastes, mesmo em condições de heterogeneidade de erros. It was observed previously a small effect of heterogeneous variances on the Bonferroni's test, Student "t" test and Tukey's test. In this paper, it was tested whether such observation was due to the fact that the error used for the tests was the treat. x block interaction mean square besides the existence of certain heterogeneous variances. The authors considered the existence of the following situations: a) two groups of variance in each experiment, and b) three kinds of errors for "t" test: treat. x block interaction mean square, specific contrast error for each comparison; and, pooled contrast error. It was simulated two hundred experiments, in groups of 50, for each of 56 combinations of four coefficients of variation, two treatment effects and seven groups of heterogeneous variances, performing a total of 11,200 simulations. In the analysis of variance, for each experiment, the following contrasts between means were considered: m1 - m3 and m2 - m3, where m3 represents the control treatments mean. The two treatment means m1 and m2 were considered "a priori" more productive than the control by 8% and 5% (effect 1) and by 17% and 12% (effect 2), respectively for m1 and m2. Thus, the one tail "t" test was applied. From the results observed, it was concluded that: a) non significant differences occurred concerning the percentages of rejections of the null hypothesis using any of the three errors considered; b) the relation among variances greater than 6 was statistically heterogeneous, when the error used was that of specific contrast error; and c) the percentages of rejections obtained were indirectly proportional to the coefficients of variation and directly proportional to the range values of contrasts between means, even in case of heterogeneous error conditions

Erro adequado para comparação de médias em casos de heterogeneidade das variâncias