

   SSasympOff {nls}                             R Documentation

   AAssyymmppttoottiicc RReeggrreessssiioonn MMooddeell wwiitthh aann OOffffsseett

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

        This `selfStart' model evaluates an alternative parame-
        terization of the asymptotic regression function and
        the gradient with respect to those parameters.  It has
        an `initial' attribute that creates initial estimates
        of the parameters `Asym', `lrc', and `c0'.

   UUssaaggee::

        SSasympOff(input, Asym, lrc, c0)

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

      input: a numeric vector of values at which to evaluate
             the model.

       Asym: a numeric parameter representing the horizontal
             asymptote on the right side (very large values of
             `input').

        lrc: a numeric parameter representing the natural loga-
             rithm of the rate constant.

         c0: a numeric parameter representing the `input' for
             which the response is zero.

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

        a numeric vector of the same length as `input'.  It is
        the value of the expression `Asym*(1 -
        exp(-exp(lrc)*(input - c0)))'.  If all of the arguments
        `Asym', `lrc', and `c0' are names of objects, the gra-
        dient matrix with respect to these names is attached as
        an attribute named `gradient'.

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

        Jose Pinheiro and Douglas Bates

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

        `nls', `selfStart'

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

        library( nls )
        data( CO2 )
        CO2.Qn1 <- CO2[CO2$Plant == "Qn1", ]
        SSasympOff( CO2.Qn1$conc, 32, -4, 43 )  # response only
        Asym <- 32; lrc <- -4; c0 <- 43
        SSasympOff( CO2.Qn1$conc, Asym, lrc, c0 ) # response and gradient
        getInitial(uptake ~ SSasymp( conc, Asym, lrc, c0), data = CO2.Qn1)
        ## Initial values are in fact the converged values
        fm1 <- nls(uptake ~ SSasymp( conc, Asym, lrc, c0), data = CO2.Qn1)
        summary(fm1)

