

   SSeettss CCoonnttrraassttss ffoorr aa FFaaccttoorr

        C(object, contr, how.many)

   AArrgguummeennttss::

     object: a factor or ordered factor

      contr: which contrasts to use. Can be a matrix with one
             row for each level of the factor or a suitable
             function like `contr.poly' or a character string
             giving the name of the function

   how.many: the number of contrasts to set, by default one
             less than `nlevels(object)'.

   DDeessccrriippttiioonn::

        Sets the `"contrasts"' attribute for the factor.

   DDeettaaiillss::

        For compatibility with S, `contr' can be `treatment',
        `helmert', `sum' or `poly' (without quotes) as short-
        hand for `contr.treatment' and so on.

   VVaalluuee::

        The factor with the  `"contrasts"' attribute set.

   AAuutthhoorr((ss))::

        B.D. Ripley

   SSeeee AAllssoo::

        `contrasts'

   EExxaammpplleess::

        data(warpbreaks)
        attach(warpbreaks)
        tens <- C(tension, poly, 1)
        attributes(tens)
        detach()
        ## tension SHOULD be an ordered factor, but as it is not we can use
        aov(breaks ~ wool + tens + tension, data=warpbreaks)

        data(esoph) # following on from help(esoph)
        model3 <- glm(cbind(ncases, ncontrols) ~ agegp + C(tobgp,, 1) +
             C(alcgp,, 1), data = esoph, family = binomial())
        summary(model3)

