drc (version 2.5-12)

EXD: 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.

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.

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 $x$.

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)