terms represent
the information about a structural model, as specified by a formula.
A
terms object is essentially a digested form of a formula that
can be used by the model fitting functions.
These objects are generated by the
terms function.
Generic functions that have methods for
"terms" objects include:
formula,
labels.
The object itself is of mode
"expression", with one element for each of
the terms, after expansion and possible simplification, and not including
the intercept.
The object also has a number of attributes:
"expression", whose elements include those in the
terms.object itself, as well as the response variable.
term.labels, to permit cross-referencing.
variables
attribute of the
terms object, and says which of the variables were among
the specials.
(See the
"Error" component in the example below.)
These objects drive all the model-fitting and are returned as a component
of the fitted object by the fitting functions
lm, etc.
terms(Y~Temp*Conc+Error(blocks), specials="Error")
# Produces the following output:
expression(Temp, Conc, Error(blocks), Temp:Conc)
attr(, "formula"):
Yield ~ Temp * Conc + Error(blocks)
attr(, "factors"):
[,1] [,2] [,3] [,4]
[1,] 0 0 0 0
[2,] 1 0 0 1
[3,] 0 1 0 1
[4,] 0 0 1 0
attr(, "order"):
[1] 1 1 1 2
attr(, "variables"):
expression(Yield, Temp, Conc, Error(blocks))
attr(, "term.labels"):
[1] "Temp" "Conc" "Error(blocks)" "Temp:Conc"
attr(, "intercept"):
[1] 1
attr(, "response"):
[1] 1
attr(, "specials"):
attr(, "specials")$Error:
[1] 4
attr(, "class"):
[1] "terms"