R: Exponential decay model

EXD {drc} | R Documentation

## Exponential decay model

### Description

Exponential decay model with or without a nonzero lower limit.

### Usage

      EXD.2(fixed = c(NA, NA), names = c("d", "e"), ...)

      EXD.3(fixed = c(NA, NA, NA), names = c("c", "d", "e"), ...)

### Arguments

`fixed` | numeric vector. Specifies which parameters are fixed and at what value they are fixed. NAs for parameter that are not fixed.
`names` | vector of character strings giving the names of the parameters (should not contain ":"). The default parameter names are: init, plateau, k.
`...` | additional arguments to be passed from the convenience functions.

### Details

The exponential decay model is a three-parameter model with mean function:

f(x) = c + (d-c)(\exp(-x/e))

The parameter init is the upper limit (attained at x=0), the parameter plateau is the lower limit reached for x going to infinity and the parameter e>0 is determining the steepness of the decay. The curve is monotonously decreasing in x.

### Value

A list of class `drcMean`, containing the mean function, the self starter function, the parameter names and other components such as derivatives and a function for calculating ED values.

### Author(s)

Christian Ritz

### References

Organisation for Economic Co-operation and Development (OECD) (2006) Current approaches in the statistical analysis of ecotoxicity data: A guidance to application - annexes, Paris: OECD (p. 80).

### See Also

Similar models giving exponential increasing curves are `AR.2` and `AR.3`.

### Examples

    ## Fitting an exponential decay model
    ryegrass.m1<-drm(rootl~conc, data=ryegrass, fct=EXD.3())

    plot(ryegrass.m1)

    summary(ryegrass.m1)

[Package drc version 3.0-1 Index]