R: Get the earth basis matrix

model.matrix.earth {earth}

R Documentation

Get the earth basis matrix

Description

Get the basis matrix of an earth model.

Usage

## S3 method for class 'earth'
model.matrix(object = stop("no 'object' argument"),
    x = NULL, subset = NULL, which.terms = NULL,
    trace = 0,
    ...,
    Env = parent.frame(),
    Callers.name = "model.matrix.earth")

Arguments

`object`	An `earth` model. This is the only required argument.
`x`	Default is NULL, meaning use the original data used to build the `earth` model (after taking the original `subset`, if any). Else `x` can be a data frame, a matrix, or a vector with length equal to a multiple of the number of columns of the original input matrix `x`. (There is some leniency here. For example, column names aren't necessary if `x` has the same number of predictors originally used to build the `earth` model.)
`subset`	Which rows to use in `x`. Default is NULL, meaning use all of `x`.
`which.terms`	Which terms to use. Default is NULL, meaning all terms in the earth model (i.e. the terms in `object$selected.terms`).
`trace`	Default 0. Set to non-zero to see which data `model.matrix.earth` is using.
`...`	Unused, but provided for generic/method consistency.
`Env`	For internal use.
`Callers.name`	For internal use (used by earth in trace messages).

Value

A basis matrix bx of the same form returned by earth. The format of bx is described in earth.object.

If x, subset, and which.terms are all NULL (the default), this function returns the model's bx. In this case, it is perhaps easier to simply use object$bx.

The matrix bx can be used as the input matrix to lm or glm, as shown below in the example. In fact, that is what earth does internally after the pruning pass — it calls lm.fit, and additionally glm if earth's glm argument is used.

Examples

data(trees)
earth.mod <- earth(Volume ~ ., data = trees)
summary(earth.mod, decomp = "none") # "none" to print terms in same order as lm.mod below

bx <- model.matrix(earth.mod)         # equivalent to bx <- earth.mod$bx
lm.mod <- lm(trees$Volume ~ bx[,-1])  # -1 to drop intercept
summary(lm.mod)                       # yields same coeffs as above summary
                                      # displayed t values are not meaningful