se.contrast               package:base               R Documentation

Standard Errors for Contrasts in Model Terms

Description:

     Returns the standard errors for one or more contrasts in an `aov'
     object.

Usage:

     se.contrast(object, ...)
     se.contrast.aov(object, contrast.obj,
       coef = contr.helmert(ncol(contrast))[, 1], data = NULL, ...) 
     se.contrast.aovlist(object, contrast.obj,
       coef = contr.helmert(ncol(contrast))[, 1], data = NULL, ...)

Arguments:

  object: A suitable fit, usually from `aov'.

contrast.obj: The contrasts for which standard errors are requested.
          This can be specified via a list or via a matrix. A single
          contrast can be specified by a list of logical vectors giving
          the cells to be contrasted. Multiple contrasts should be
          specified by a matrix as returned by `contrasts'.

    coef: Used when `{contrast.obj}' is a list; it should be a vector
          of the same length as the list with zero sum. The default
          value is the first Helmert contrast, which contrasts the
          first and second cell means specified by the list.

    data: The data frame used to evaluate `contrast.obj'.

     ...: further arguments passed to or from other methods.

Details:

     Contrasts are usually used to test if certain means are
     significantly different; it can be easier to use `se.contrast'
     than compute directly with the coefficients.

     In multistratum models, the contrasts can appear in more than one
     stratum; the contrast and standard error are computed in the
     lowest stratum and adjusted for efficiencies and comparisons
     between strata.

Value:

     A vector giving the standard errors for each contrast.

Author(s):

     B.D. Ripley

See Also:

     `contrasts', `model.tables'

Examples:

     ## From Venables and Ripley (1997) p.210.
     N <- c(0,1,0,1,1,1,0,0,0,1,1,0,1,1,0,0,1,0,1,0,1,1,0,0)
     P <- c(1,1,0,0,0,1,0,1,1,1,0,0,0,1,0,1,1,0,0,1,0,1,1,0)
     K <- c(1,0,0,1,0,1,1,0,0,1,0,1,0,1,1,0,0,0,1,1,1,0,1,0)
     yield <- c(49.5,62.8,46.8,57.0,59.8,58.5,55.5,56.0,62.8,55.8,69.5,
     55.0, 62.0,48.8,45.5,44.2,52.0,51.5,49.8,48.8,57.2,59.0,53.2,56.0)

     npk <- data.frame(block=gl(6,4), N=factor(N), P=factor(P),
                       K=factor(K), yield=yield)
     options(contrasts=c("contr.treatment", "contr.poly"))
     npk.aov1 <- aov(yield ~ block + N + K, npk)
     se.contrast(npk.aov1, list(N=="0", N=="1"), data=npk)

