drc (version 0.8-2)

logistic: The logistic function

Description

'logistic' provides a very general way of specifying logistic dose response functions, under various constraints on the parameters.

Usage

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)

Arguments

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!

Value

• The value returned by the 'logistic' 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 ED function.
• sifct
  The SI function.

Details

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.

References

Finney, D. J. (1979) Bioassay and the Practise of Statistical Inference, Int. Statist. Rev., 47, 1--12.

See Also

For convenience several special cases of the function 'logistic' are available: l3, l4 and l5.

Examples

## 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)