Contemporary Groups Are Always Random

Abstract

Since 1970 when Dr. Henderson introduced the Northeast AI Sire Comparison model and methods to the world, herd-year-season effects (contemporary groups (CG) in that model)have been treated as fixed effects. This was based on Dr. Henderson’s selection bias theory in which the bias being considered was that the true sire genetic values were associated with the true herd-year-season (HYS) effects. That is, herds with better management effects on their cows tended to use the genetically higher valued sires on their cows to produce the first lactation daughters in a herd. Henderson naturally thought of herd-year-season effects as random (as discussed in a video from Kyoto, Japan in 1985), but his selection theory told him that either HYS or sire effects needed to be treated as fixed effects to eliminate the bias from the model. Because sire genetic values were more important at the time, he chose to make HYS fixed rather than sire effects. Subsequently,every genetic evaluation system has used fixed CG, even though it does not make sense to do so in all situations.