

   HHuuyynn--FFeellddtt CCoorrrreellaattiioonn SSttrruuccttuurree

        corHF(value, form)

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

      value: an optional vector with the parameter values.
             Default is `numeric(0)', which results in a vector
             of zeros of appropriate dimension being assigned
             to the parameters when `object' is initialized
             (corresponding to an identity correlation struc-
             ture).

       form: a one sided formula of the form `~ t', or code{~ t
             | g}, specifying a time covariate `t' and,
             optionally, a grouping factor `g'. A covariate for
             this correlation structure must be integer valued.
             When a grouping factor is present in `form', the
             correlation structure is assumed to apply only to
             observations within the same grouping level;
             observations with different grouping levels are
             assumed to be uncorrelated. Defaults to `~ 1',
             which corresponds to using the order of the obser-
             vations in the data as a covariate, and no groups.

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

        This function is a constructor for the `corHF' class,
        representing a correlation structure satisfying the
        Huyn-Feldt conditions
        r(i,j)=g(i)+g(j)+(1-2*g(i))*d(i,j), where r(i,j)
        denotes the correlation between observations `i' and
        `j', g(i) and g(j) are the corresponding parameter val-
        ues, and d(i,j) is an indicator variable assuming value
        1 when i=j and 0 otherwise. Objects created using this
        constructor need to be later initialized using the
        appropriate `initialize' method.

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

        an object of class `corHF', representing an  Huyn-Feldt
        correlation structure.

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

        Jose Pinheiro and Douglas Bates

   RReeffeerreenncceess::

        Milliken, G. A. and Johnson, D. E. (1992) "Analysis of
        Messy Data, Volume I: Designed Experiments", Van Nos-
        trand Reinhold.

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

        `initialize.corHF'

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

        library(lme)
        ## covariate is observation order and grouping factor is Subject
        cs1 <- corHF(form = ~ 1 | Subject)

