The two-parameter log-logistic function
'LL.2' and 'LL2.2' provide 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.
LL.2(upper = 1, fixed = c(NA, NA), names = c("b", "e"), ...)
l2(upper = 1, fixed = c(NA, NA), names = c("b", "e"), ...)
LL2.2(upper = 1, fixed = c(NA, NA), names = c("b", "e"), ...)upper |
numeric value. The fixed, upper limit in the model. Default is 1. |
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 |
The two-parameter log-logistic function is given by the expression
f(x) = \frac{1}{1+\exp(b(\log(x)-\log(e)))}
or in another parameterisation
f(x) = \frac{1}{1+\exp(b(\log(x)-e))}
The model function is symmetric about the inflection point (e).
See llogistic.
This function is for use with the function drm.
Christian Ritz
## Fitting a two-parameter logistic model ## to binomial responses (a logit model) earthworms.m1 <- drm(number/total~dose, weights=total, data = earthworms, fct = LL.2(), type = "binomial") plot(earthworms.m1) # not fitting at the upper limit!
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