twophase.RdThe two-phase dose-response model is a combination of log-logistic models that should be useful for describing more complex dose-response patterns.
twophase(fixed = c(NA, NA, NA, NA, NA, NA, NA), names = c("b1", "c1", "d1", "e1", "b2", "d2", "e2"), fctName, fctText)
| fixed | numeric vector specifying which parameters are fixed and at what value they are fixed. NAs are used for parameters that are not fixed. |
|---|---|
| names | a vector of character strings giving the names of the parameters (should not contain ":"). The default is reasonable (see under 'Usage'). |
| fctName | optional character string used internally by convenience functions. |
| fctText | optional character string used internally by convenience functions. |
Following Groot et al (1996) the two-phase model function is defined as follows
$$ f(x) = c + \frac{d1-c}{1+\exp(b1(\log(x)-\log(e1)))} + \frac{d2}{1+\exp(b2(\log(x)-\log(e2)))}$$
For each of the two phases, the parameters have the same interpretation as in the ordinary log-logistic model.
The value returned is a list containing the nonlinear function, the self starter function and the parameter names.
Groot, J. C. J., Cone, J. W., Williams, B. A., Debersaques, F. M. A., Lantinga, E. A. (1996) Multiphasic analysis of gas production kinetics for in vitro fermentation of ruminant feeds, Animal Feed Science Technology, 64, 77--89.
The basic component in the two-phase model is the log-logistic model
llogistic.