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The Dairy Herd Improvement Association (DHIA) age factors of Kendrick and MeDaniel were compared using 10,620 first lactations made in Michigan Holstein herds during the period 1961-63. The effectiveness of the alternative age adjustments was judged by the impact on sire ranking and on the statistical properties of the standardized records. Bulls were found to rank almost identically, regardless of the manner of age adjustment. Rank correlations between proofs based on actual and age-corrected records were about .95. There were significant regressions of age-adjusted production on age, for both sets of factors. It was concluded that standard age adjustments may not be satisfactory in research applications requiring a high degree of accuracy.
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Miller et al. (1968) studied this question.
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