The four-parameter log-logistic function
'LL.4' and 'LL2.4' provide the four-parameter log-logistic function, self starter function, names of the parameters and, optionally, first and second derivatives for a faster estimation.
LL.4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), ...)
l4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), ...)
LL2.4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), ...)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 four-parameter log-logistic function is given by the expression
f(x) = c + \frac{d-c}{1+\exp(b(\log(x)-\log(e)))}
or in another parameterisation (converting the term \log(e) into a parameter)
f(x) = c + \frac{d-c}{1+\exp(b(\log(x)-\tilde{e}))}
The function is symmetric about the inflection point (e).
See llogistic.
This function is for use with the function drm.
Christian Ritz and Jens C. Streibig
Seber, G. A. F. and Wild, C. J (1989) Nonlinear Regression, New York: Wiley \& Sons (p. 330).
spinach.m1 <- drm(SLOPE~DOSE, CURVE, data = spinach, fct = LL.4()) spinach.m1
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