drc (version 2.5-12)

cedergreen: The Cedergreen-Ritz-Streibig model

Description

'cedergreen' provides a very general way of specifying then Cedergreen-Ritz-Streibig modified log-logistic model for describing hormesis, under various constraints on the parameters. CRS.6 is the extension of link{cedergreen} with freely varying alpha parameter. For u-shaped hormesis data 'ucedergreen' provides a very general way of specifying the Cedergreen-Ritz-Streibig modified log-logistic model, under various constraints on the parameters.

Usage

cedergreen(fixed = c(NA, NA, NA, NA, NA), 
  names = c("b", "c", "d", "e", "f"), 
  method = c("1", "2", "3", "4"), ssfct = NULL,
  alpha, fctName, fctText)
  
  CRS.6(fixed = c(NA, NA, NA, NA, NA, NA), 
  names = c("b","c","d","e","f","g"),
  method = c("1", "2", "3", "4"), ssfct = NULL)
  
  ucedergreen(fixed = c(NA, NA, NA, NA, NA), 
  names = c("b", "c", "d", "e", "f"), 
  method = c("1", "2", "3", "4"), ssfct = NULL,
  alpha)

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.

alpha

numeric value between 0 and 1, reflecting the steepness of the hormesis peak. This argument needs to be specified.

fctName

optional character string used internally by convenience functions.

fctText

optional character string used internally by convenience functions.

Value

• The value returned is a list containing the non-linear function, the self starter function and the parameter names.

concept

hormesis hormetic effect initial stimulation u-shaped

Details

The model is given by the expression $$f(x) = c + \frac{d-c+f exp(-1/(x^{\alpha}))}{1+exp(b(log(x)-log(e)))}$$ which is a five-parameter model (alpha is fixed or freely varying). Not all features (eg EC/ED calculation) are available for the model with freely varying alpha. It is a modification of the four-parameter logistic curve to take hormesis into account. The u-shaped model is given by the expression $$f(x) = cd - \frac{d-c+f \exp(-1/x^{\alpha})}{1+\exp(b(\log(x)-\log(e)))}$$

References

Cedergreen, N. and Ritz, C. and Streibig, J. C. (2005) Improved empirical models describing hormesis, Environmental Toxicology and Chemistry 24, 3166--3172.

See Also

For fixed alpha, several special cases are handled by the following convenience functions CRS.4a, CRS.4b, CRS.4c, CRS.5a, CRS.5b, CRS.5c, UCRS.4a, UCRS.4b, UCRS.4c, UCRS.5a, UCRS.5b, UCRS.5c where a, b and c correspond to the pre-specified alpha values 1, 0.5 and 0.25, respectively.

Examples

## Estimating CRS model with alpha unknown
lettuce.crsm1 <- drm(weight~conc, data = lettuce, fct = CRS.6())
summary(lettuce.crsm1)
plot(lettuce.crsm1)  # oops: not increasing until hormesis peak