richards {drc}R Documentation

Richards model

Description

'richards'

Usage

  richards(fixed = c(NA, NA, NA, NA, NA), names = c("c", "d", "delta", "kappa", "gamma"))

Arguments

fixed numeric vector. Specifies which parameters are fixed and at what value they are fixed. NAs for parameter that are not fixed.
names a vector of character strings giving the names of the parameters (should not contain ":"). The order of the parameters is: c, d, delta, kappa, gamma (see under 'Details').

Details

The model is given by the expression

f(x) = c + (d-c) ( 1 + (delta-1)exp(-kappa(log(x)-log(gamma))) )^(1/(1-delta))

It is a five-parameter model suitable for asymmetric dose-response relationships. In fact it is a reparameterisation of the five-parameter log-logistic model.

For delta=2 the model simplifies the four-parameter log-logistic model.

Value

An object containing with all components needed in multdrc.

Note

It may be difficult to get this model to converge.

Author(s)

Christian Ritz

References

Seber, G. A. F. and Wild, C. J. (1989) Nonlinear Regression, New York: Wiley & Sons (pp. 332–337).

Examples


## Fitting Richards model
m1 <- multdrc(ryegrass, fct = richards())


[Package drc version 1.4-2 Index]