Exploration & description/Observational units

In mixed model analysis we have different types of units occurring at different layers – namely in this example: lambs, ewes, rams. The investigational or observational units defined within layers are assumed to be chosen independently of one another; usually they are chosen at ´random´. They will therefore be random effects in our mixed model.

We can do exactly the same for ewes. As the selection process is being carried out at the same time as the rams, the mixed model tree is formed in parallel.

A ewe's age is an attribute that we feel may influence weaning weight.