richards {drc} | R Documentation |
'richards'
richards(fixed = c(NA, NA, NA, NA, NA), names = c("c", "d", "delta", "kappa", "gamma"))
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'). |
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.
An object containing with all components needed in multdrc
.
It may be difficult to get this model to converge.
Christian Ritz
Seber, G. A. F. and Wild, C. J. (1989) Nonlinear Regression, New York: Wiley & Sons (pp. 332–337).
## Fitting Richards model m1 <- multdrc(ryegrass, fct = richards())