contrast {base}R Documentation

Contrast Matrices

Description

Return a matrix of contrasts.

Usage

contr.helmert(n, contrasts = TRUE)
contr.poly(n, contrasts = TRUE)
contr.sum(n, contrasts = TRUE)
contr.treatment(n, base = 1, contrasts = TRUE)

Arguments

n a vector of levels for a factor, or the number of levels.
contrasts a logical indicating whether contrasts should be computed.
base an integer specifying which group is considered the baseline group.

Details

These functions are used for creating contrast matrices for use in fitting analysis of variance and regression models. The columns of the resulting matrices contain contrasts which can be used for coding a factor with n levels. The returned value contains the computed contrasts. If the argument contrasts is FALSE then an additional column of ones is prepended to the matrix.

Note that as from R version 0.62.2, contr.poly returns contrasts based on orthogonal (rather than raw) polynomials.

Value

A matrix with n rows and k columns, with k=n-1 if contrasts is TRUE and k=n if contrasts is FALSE.

See Also

contrasts, C, and aov, glm, lm.

Examples

(cH <- contr.helmert(4))
apply(cH, 2,sum)# column sums are 0!
crossprod(cH)# diagonal -- columns are orthogonal

(cT <- contr.treatment(5))
all(crossprod(cT) == diag(4))# TRUE: even orthonormal

(cP <- contr.poly(3))# Linear and Quadratic
zapsmall(crossprod(cP), dig=15) # orthonormal up to fuzz

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