C
Assign Contrasts to a Factor

### Description

Assign contrasts to a factor in a model formula or create a new factor for use in statistical modelling functions.

### Usage

    C(object, contr, how.many)

### Arguments

object | a factor or an ordered factor.
contr | a function, a matrix, or character string that specifies the contrasts to use. If a matrix, the matrix must have as many rows as there are levels in the factor. If you specify a character string, you can submit one of the following five standard names: (helmert, poly, treatment, sum), or SAS. The default is the first element of the contrasts option for factors and the second element of the option for ordered factors.
how.many | an integer that specifies the number of contrasts to be assigned to the factor. The default is nlevels(object)-1.

### Details

If you set how.many to less than nlevels(object)-1 in a model formula you are asserting that the coefficients for the remaining factors are either known to be negligible or else should be aliased with other coefficients.
Value
returns the input factor with an added contrasts attribute.
See Also
contrasts, contr.sum, contr.treatment, aov, factor, ordered.
Examples

    data(warpbreaks)
    # treat 2nd level of tension as baseline for just the next analysis
    lm(breaks ~ wool + C(tension, treatment, base=2), data=warpbreaks)
    # treat last level as baseline for all future analyses
    contrasts(warpbreaks$tension) <- "contr.SAS"
    lm(breaks ~ wool + tension, data=warpbreaks)
    contrasts(warpbreaks$tension) <- NULL

Package stats version 6.1.5-18
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