drc (version 1.0-3)

w4: 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!

Value

• See weibull.

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 $\exp(e)$.

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)