Summary
This case study continues the
analysis of differences in weaning weight between indigenous
genotypes of sheep which was started in Case Study 3. In the previous case study a model containing
fixed effects for lamb genotype, year of birth, sex, age at weaning
and age of dam was fitted by the method of general least squares. Here we
extend the model by introducing random effects for sire and dam and
use the method of restricted maximum likelihood (REML) to fit the
mixed model. The case study explores the multilevel structure of the
data and shows how the different layers can be expressed
diagrammatically in the form of a ´mixed model tree´.
The outputs produced by REML are described and compared with
outputs produced by the method of general least squares. Although
the presentations of results are different, analyses of variance and
parameter estimates and standard errors are shown to be the same
when no random terms are included in the model. Random terms for ram
and ewe are then added to the statistical model. The interpretation
and significance of their effects are discussed. The use of R for the analysis of these data is
illustrated as well as GenStat.
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