terms.object {stats} | R Documentation |
Description of Terms Objects
Description
An object of class terms
holds information about a
model. Usually the model was specified in terms of a
formula
and that formula was used to determine the terms
object.
Details
The object itself is simply the result of terms.formula(<formula>)
.
It has a number of attributes
and they are used to construct the model frame:
factors
An integer matrix of variables by terms showing which variables appear in which terms. The entries are
- 0
if the variable does not occur in the term,
- 1
if it does occur and should be coded by contrasts, and
- 2
if it occurs and should be coded via dummy variables for all levels (as when a lower-order term is missing).
Note that variables in main effects always receive 1, even if the intercept is missing (in which case the first one should be coded with dummy variables). If there are no terms other than an intercept and offsets, this is
integer(0)
.term.labels
A character vector containing the labels for each of the terms in the model, except for offsets. Note that these are after possible re-ordering of terms.
Non-syntactic names will be quoted by backticks: this makes it easier to re-construct the formula from the term labels.
variables
A call to
list
of the variables in the model.intercept
Either 0, indicating no intercept is to be fit, or 1 indicating that an intercept is to be fit.
order
A vector of the same length as
term.labels
indicating the order of interaction for each term.response
The index of the variable (in variables) of the response (the left hand side of the formula). Zero, if there is no response.
offset
If the model contains
offset
terms there is anoffset
attribute indicating which variable(s) are offsetsspecials
If a
specials
argument was given toterms.formula
there is aspecials
attribute, a pairlist of vectors (one for each specified special function) giving numeric indices of the arguments of the list returned as thevariables
attribute which contain these special functions.dataClasses
optional. A named character vector giving the classes (as given by
.MFclass
) of the variables used in a fit.predvars
optional. An expression to help in computing predictions at new covariate values; see
makepredictcall
.
The object has class c("terms", "formula")
.
Note
These objects are different from those found in S. In particular
there is no formula
attribute: instead the object is itself a
formula. (Thus, the mode of a terms object is different.)
Examples of the specials
argument can be seen in the
aov
and coxph
functions, the
latter from package survival.
See Also
Examples
## use of specials (as used for gam() in packages mgcv and gam)
(tf <- terms(y ~ x + x:z + s(x), specials = "s"))
## Note that the "factors" attribute has variables as row names
## and term labels as column names, both as character vectors.
attr(tf, "specials") # index 's' variable(s)
rownames(attr(tf, "factors"))[attr(tf, "specials")$s]
## we can keep the order by
terms(y ~ x + x:z + s(x), specials = "s", keep.order = TRUE)