drc (version 1.0-3)

l2: The two-parameter logistic function

Description

'l2' provides the two-parameter logistic function where the lower limit is fixed at 0 and the upper limit is fixed at 1, mostly suitable for binomial/quantal responses.

Usage

l2(fixed = c(NA, NA), names = c("b", "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 logistic.

Details

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

See Also

Related functions are l3, l4, l5 and the more general logistic.

Examples

## Fitting a two-parameter logistic model 
##  to binomial responses (a logit model)
model1 <- multdrc(number/total~dose, weights=total, 
data=earthworms, fct = l2(), type = "binomial")
plot(model1)  # not fitting at the upper limit!

rm(model1)