SynopsisA method is presented by which the sampling variance of all heritability estimates derived from ratios involving variance components may be found.

THE relative merits of individual selection and family selection in breeding for traits of incomplete heritability were first discussed by Lush (i 947) and, more recently, with particular reference to poultry breeding, by Lerner (r 950). The general conclusion is that for traits of low heritability, selection of complete families of full or half-sibs without regard to individual performance is more efficient than selection on the basis of individual phenotypes. For higher values of heritability the situation may be reversed, but in all cases maximum efficiency can be obtained by selection on the basis of a combination of family average and individual record.In poultry and other animal populations the family classification The first and second of these systems are discussed in detail by Lerner (1950, ch. XI) but neither of the others have been investigated, the nearest approach being the comparisons by Lerner and Lush of the relative merits of mass selection and half-sib family selection. These comparisons, however, neglect the full-sib relationship which exists between many pairs of a half-sib group in the above hierarchy. It is evident that the first three of the above systems are particular 93

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