

   SSttaannddaarrdd EErrrroorrss ffoorr CCoonnttrraassttss iinn MMooddeell TTeerrmmss

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

   AArrgguummeennttss::

     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 con-
             trast, which contrasts the first and second cell
             means specified by the list.

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

   DDeessccrriippttiioonn::

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

   DDeettaaiillss::

        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 coeffi-
        cients.

        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.

   VVaalluuee::

        A vector giving the standard errors for each contrast.

   AAuutthhoorr((ss))::

        B.D. Ripley

   EExxaammpplleess::

        ## 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)

