| 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)