w4 {drc} | R Documentation |
'w4' provides the four-parameter Weibull function, self starter function and names of the parameters.
w4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), useDer = FALSE)
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. The default is reasonable. |
useDer |
logical. If TRUE derivatives are supplied, otherwise they are not supplied. Not implemented! |
The four-parameter Weibull model is given by the expression
f(x) = c + (d-c)exp(-exp(b(log(x)-e))).
The function is asymmetric about the inflection point, that is the parameter exp(e).
See weibull
.
This function is for use with the function multdrc
.
Christian Ritz
Seber, G. A. F. and Wild, C. J (1989) Nonlinear Regression, New York: Wiley & Sons (pp. 330–331).
Setting c=0 yields w3
. A more flexible function, allowing fixing or constraining parameters, is weibull
.
## Fitting a four-parameter Weibull model model1 <- multdrc(rgr~dose, pct, data = TM, fct = w4()) summary(model1) rm(model1)