drc (version 1.0-3)

l4: The four-parameter logistic function

Description

'l4' provides the four-parameter logistic function, self starter function, names of the parameters and, optionally, first and second derivatives for a faster estimation.

Usage

l4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), useDer = FALSE, w = 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, if FALSE (default) they are not supplied.

w

logical. If TRUE weighted regression is applied to determine the start values.

Value

• See logistic.

Details

The four-parameter logistic function is given by the expression $$f(x) = c + \frac{d-c}{1+\exp(b(\log(x)-\log(e)))}$$ The function is symmetric about the inflection point ($e$).

References

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

See Also

Setting $c=0$ yields l3. See also l5.

Examples

model1 <- multdrc(SLOPE~DOSE, CURVE, data=PestSci)
model1

rm(model1)