Genetic improvement in a flock depends on the producer’s ability to select breeding sheep that are genetically superior for traits of economic importance. This is complicated by the fact that an animal’s own performance is not always a true indicator of its genetic potential as a parent.
Aaron, Debra K., "Sheep Breeding: Heritability, EBVs, EPDs and the NSIP" (2014). Agriculture and Natural Resources Publications. Paper 154.