R: Construct design matrices

model.matrix {base}

R Documentation

Construct design matrices

Description

model.matrix creates a design matrix. model.weights extracts the weights.

Usage

model.matrix   (object, ...)
model.matrix.lm(object, ...)
model.matrix.default(formula, data, contrasts.arg = NULL, xlev = NULL)
model.weights(x)

Arguments

formula

a model formula or terms object.

data

a data frame created with model.frame.

contrasts.arg

A list, whose entries are contrasts suitable for input to the contrasts function and whose names are the names of columns of data containing factors.

xlev

to be used as argument of model.frame if data has no "terms" attribute.

x

a modle frame.

Value

The design matrix for a regression model with the specified formula and data.

References

Chambers and Hastie: "Statistical Models in S".

Examples

data(trees)
ff <- log(Volume) ~ log(Height) + log(Girth)
str(m <- model.frame(ff, trees))
mat <- model.matrix(ff, m)

dd <- data.frame(a = gl(3,4), b = gl(4,1,12))# balanced 2-way
options("contrasts")
model.matrix(~a+b,dd)
model.matrix(~a+b,dd,contrasts=list(a="contr.sum"))
model.matrix(~a+b,dd,contrasts=list(a="contr.sum",b="contr.poly"))
m.orth <- model.matrix(~a+b,dd, contrasts=list(a="contr.helmert"))
crossprod(m.orth)# m.orth is  ALMOST  orthogonal

[Package base version 0.90 ]