| logistic {drc} | R Documentation |
'logistic' provides a very general way of specifying logistic dose response functions, under various constraints on the parameters.
logistic(lowerc = c(-Inf, -Inf, -Inf, -Inf, -Inf),
upperc = c(Inf, Inf, Inf, Inf, Inf), fixed = c(NA, NA, NA, NA, NA),
names = c("b", "c", "d", "e", "f"), scaleDose = TRUE, useDer = FALSE)
lowerc |
numeric vector. The lower bound on parameters. Default is minus infinity. |
upperc |
numeric vector. The upper bound on parameters. Default is plus infinity. |
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 (should not contain ":"). The default is reasonable (see under 'Usage'). The order of the parameters is: b, c, d, e, f (see under 'Details'). |
scaleDose |
logical. If TRUE dose values are scaled around 1 during estimation; this is required for datasets where all dose values are small. |
useDer |
logical. If TRUE derivatives are supplied, otherwise they are not supplied. Not yet implemented! |
The default arguments yields the five-parameter logistic function given by the expression
f(x) = c + frac{d-c}{(1+exp(b(log(x)-log(e))))^f}
If the parameter f is different from 1 then the function is asymmetric.
The value returned is a list with the following components
fct |
The dose response function. |
ssfct |
The self starter function. |
deriv1 |
The first derivative. |
deriv2 |
The second derivative. |
lowerc |
The lower bounds on the parameters. |
upperc |
The upper bounds on the parameters. |
edfct |
The function used to calculate ED values. |
sifct |
The function used to calculate SI values. |
The functions are for use with the function multdrc.
Christian Ritz
Finney, D. J. (1979) Bioassay and the Practise of Statistical Inference, Int. Statist. Rev., 47, 1–12.
Seber, G. A. F. and Wild, C. J (1989) Nonlinear Regression, New York: Wiley & Sons (p. 330).
For convenience several special cases of the function 'logistic' are available:
l2, l3, l4 and l5.
## A four-parameter logistic model with b fixed at 1 model1 <- multdrc(FA, fct=logistic(fixed=c(1, NA, NA, NA, 1))) summary(model1) ## Can also be specified using the function 'l4(fixed=c(1,NA,NA,NA))' ## A four-parameter logistic model with the constraint b>3 model2 <- multdrc(FA, fct=logistic(fixed=c(NA, NA, NA, NA, 1), lowerc=c(3, -Inf, -Inf, -Inf, -Inf)), control=mdControl(constr=TRUE)) summary(model2) rm(model1, model2)