SSgompertz                package:nls                R Documentation

Gompertz Growth Model

Description:

     This `selfStart' model evaluates the Gompertz growth model and its
     gradient.  It has an `initial' attribute that creates initial
     estimates of the parameters `Asym', `b2', and `b3'.

Usage:

     SSgompertz(x, Asym, b2, b3)

Arguments:

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

    Asym: a numeric parameter representing the asymptote.

      b2: a numeric parameter related to the value of the function at
          `x = 0'

      b3: a numeric parameter related to the scale the `x' axis.

Value:

     a numeric vector of the same length as `input'.  It is the value
     of the expression `Asym*exp(-b2*b3^x)'.  If all of the arguments
     `Asym', `b2', and `b3' are names of objects the gradient matrix
     with respect to these names is attached as an attribute named
     `gradient'.

Author(s):

     Douglas Bates

See Also:

     `nls', `selfStart'

Examples:

     data(DNase )
     DNase.1 <- subset(DNase, Run == 1)
     SSlogis(log(DNase.1$conc), 4.5, 2.3, 0.7)  # response only
     Asym <- 4.5; b2 <- 2.3; b3 <- 0.7
     SSgompertz(log(DNase.1$conc), Asym, b2, b3 ) # response and gradient
     getInitial(density ~ SSgompertz(log(conc), Asym, b2, b3),
                data = DNase.1)
     ## Initial values are in fact the converged values
     fm1 <- nls(density ~ SSgompertz(log(conc), Asym, b2, b3),
                data = DNase.1)
     summary(fm1)

