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)