drc (version 1.2-0)

LL.2: The two-parameter log-logistic function

Description

'LL.2' provides the two-parameter log-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

LL.2(fixed = c(NA, NA), names = c("b", "e"), ...)
  
  l2(fixed = c(NA, NA), names = c("b", "e"), ...)

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.

...

Additional arguments (see llogistic).

Value

• See llogistic.

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 LL.3, LL.4, LL.5 and the more general llogistic.

Examples

## Fitting a two-parameter logistic model 
##  to binomial responses (a logit model)
model1 <- multdrc(number/total~dose, weights=total, 
data=earthworms, fct = LL.2(), type = "binomial")
plot(model1)  # not fitting at the upper limit!