# Model functions

## mu

mu
is used
to fit the intercept/constant term. It is normally present and listed first in the
model. It should be present in the model
if there are no other fixed factors or if
all fixed terms are covariates or contrasts except in the
special case of regression through the origin.
## Trait

Trait
is used with multivariate data to
fit the individual trait means. It is formally equivalent to mu
but Trait is a more natural label for use with
multivariate data. It is interacted with other factors to estimate their effects for all traits.
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