R: The logistic model

logistic {drc} | R Documentation

## The logistic model

### Description

The general asymmetric five-parameter logistic model for describing dose-response relationships.

### Usage

      logistic(fixed = c(NA, NA, NA, NA, NA), names = c("b", "c", "d", "e", "f"),
      method = c("1", "2", "3", "4"), ssfct = NULL,
      fctName, fctText)

      L.3(fixed = c(NA, NA, NA), names = c("b", "d", "e"), ...)
      L.4(fixed = c(NA, NA, NA, NA), names = c("b", "c", "d", "e"), ...)
      L.5(fixed = c(NA, NA, NA, NA, NA), names = c("b", "c", "d", "e", "f"), ...)

### Arguments

`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 order of the parameters is: b, c, d, e, f (see under 'Details').
`method` | character string indicating the self starter function to use.
`ssfct` | a self starter function to be used.
`fctName` | optional character string used internally by convenience functions.
`fctText` | optional character string used internally by convenience functions.
`...` | Additional arguments (see `llogistic`).

### Details

The default arguments yields the five-parameter logistic mean function given by the expression

f(x) = c + \frac{d-c}{(1+\exp(b(x - e)))^f}

The model is different from the log-logistic models `llogistic` and `llogistic2` where the term

log(x)

is used instead of

x

.

The model is sometimes referred to as the Boltzmann model.

### Value

The value returned is a list containing the nonlinear function, the self starter function and the parameter names.

### Author(s)

Christian Ritz

### Examples

    ## Fitting the four-parameter logistic model
    ryegrass.m1 <- drm(rootl ~ conc, data = ryegrass, fct = L.4())
    summary(ryegrass.m1)

    ## Fitting an asymmetric logistic model
    ##  requires installing the package 'NISTnls'
    # Ratkowsky3.m1 <- drm(y~x, data = Ratkowsky3,
    # fct = L.5(fixed = c(NA, 0, NA, NA, NA)))
    # plot(Ratkowsky3.m1)
    # summary(Ratkowsky3.m1)
    ## okay agreement with NIST values
    ##  for the two parameters that are the same

[Package drc version 3.0-1 Index]