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.

Rams and ewes are mated both within and across breeds to produce their offspring. These offspring are the investigational units at the next layer down shown together with a list of fixed effects or attributes that might be considered for each lamb.

Breed differences, however, are assessed relative to the average variation among rams and ewes within breed at the top layer.