Van Vleck L D
Department of Animal Science, Cornell University, Ithaca, NY 14853.
J Dairy Sci. 1987 Nov;70(11):2456-64. doi: 10.3168/jds.S0022-0302(87)80309-0.
Contemporary groups are used to remove biases from genetic evaluations due to differential effects such as management associated with the grouping. Numerous groups, however, can result in small numbers of records per subclass with associated loss of effective number of daughters for sire evaluation and increased prediction error variance. Thus, in practice, mean square error, bias squared plus prediction error variance, may be more meaningful than bias alone or prediction error variance. Considering contemporary groups as fixed removes bias due to association between effects corresponding to contemporary groups and sires. If contemporary groups are considered random, then effective number of daughters is increased at the expense of possible bias. Various compromises may be effective for increasing genetic gain. Arbitrary definition of contemporary groups can include herd-year-season of freshening, lactation number, registered or nonregistered, sampling or postsampling daughters, and special treatments among others. The assumption of homogeneous genetic and residual variances is likely to be incorrect. Alternative methods include simple transformations, a two-step transformation, and multiple trait modeling. Multiple trait analyses may include the assumption of genetic correlations of unity, common genetic and heterogeneous residual variances, and joint estimation of genetic values and variances.
当代组用于消除遗传评估中的偏差，这些偏差源于诸如与分组相关的管理等差异效应。然而，大量的组可能导致每个亚类的记录数量较少， sire评估中有效女儿数量减少，预测误差方差增加。因此，在实际应用中，均方误差（偏差平方加上预测误差方差）可能比单独的偏差或预测误差方差更有意义。将当代组视为固定因素可以消除由于当代组效应与 sire之间的关联而产生的偏差。如果将当代组视为随机因素，那么有效女儿数量会增加，但可能会产生偏差。各种折衷方法可能对增加遗传增益有效。当代组的任意定义可以包括产犊的畜群-年份-季节、泌乳次数、注册或未注册、抽样或抽样后女儿以及特殊处理等。遗传方差和残差方差齐性的假设可能是不正确的。替代方法包括简单变换、两步变换和多性状建模。多性状分析可能包括遗传相关为1的假设、共同遗传和异质残差方差以及遗传值和方差的联合估计。
