gompertz {drc} | R Documentation |
This function provides a very general way of specifying the mean function of the decreasing or incresing Gompertz dose-response or growth curve models.
gompertz(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), fctName, fctText)
fixed |
numeric vector. Specifies which parameters are fixed and at what value they are fixed. NAs for parameter that are not fixed. |
names |
vector of character strings giving the names of the parameters (should not contain ":"). The default is reasonable (see under 'Usage'). The order of the parameters is: b, c, d, e (see under 'Details' for the precise meaning of each parameter). |
fctName |
character string used internally by convenience functions (optional). |
fctText |
character string used internally by convenience functions (optional). |
The Gompertz model is given by the mean function
f(x) = c + (d-c)(exp(-exp(b(x-e))))
and it is a dose-response/growth curve on the entire real axis, that is it is not limited to non-negative values even though this is the range for most dose-response and growth data. One consequence is that the curve needs not reach the lower asymptote at dose 0.
If
b<0
the mean function is increasing and it is decreasing for
b>0
. The decreasing Gompertz model is not a well-defined dose-response model and other dose-response models such as the Weibull models should be used instead.
Various re-parameterisations of the model are used in practice.
The value returned is a list containing the non-linear function, the self starter function and the parameter names.
The function is for use with the function drm
, but typically the convenience functions
link{G.2}
, link{G.3}
, link{G.3u}
, and link{G.4}
should be used.
Christian Ritz
Seber, G. A. F. and Wild, C. J. (1989) Nonlinear Regression, New York: Wiley & Sons (p. 331).
The Weibull model weibull1
is closely related to the Gompertz model.
bg.m1 <- drm(weightInf~DAE, data = beetGrowth, fct = G.3()) summary(bg.m1) plot(bg.m1)