w4 {drc}R Documentation

The four-parameter Weibull function

Description

'w4' provides the four-parameter Weibull function, self starter function and names of the parameters.

Usage

w4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), useDer = FALSE)

Arguments

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!

Details

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).

Value

See weibull.

Note

This function is for use with the function multdrc.

Author(s)

Christian Ritz

References

Seber, G. A. F. and Wild, C. J (1989) Nonlinear Regression, New York: Wiley & Sons (pp. 330–331).

See Also

Setting c=0 yields w3. A more flexible function, allowing fixing or constraining parameters, is weibull.

Examples


## Fitting a four-parameter Weibull model
model1 <- multdrc(rgr~dose, pct, data = TM, fct = w4())
summary(model1)

rm(model1)

[Package drc version 0.9-0 Index]