Contrast Matrices

Usage

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

Value

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

The base argument to contr.treatment() allows you to specify which group is considered the baseline group.

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.

See Also

contrasts, glm, lm.


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