mixture: Fitting mixture models

Description

'mixture' fits a concentration addition, Hewlett or Voelund model to data from mixture toxicity experiments.

Usage

mixture(object, model = c("CA", "Hewlett", "Voelund"), start, startm, control = drmc())

Arguments

object

object of class 'drc' corresponding to the model with freely varying EC50 values.

model

character string. It can be "CA", "Hewlett" or "Voelund".

start

optional numeric vector supplying starting values for all parameters in the mixture model.

startm

optional numeric vector supplying the lambda parameter in the Hewlett model or the eta parameters (two parameters) in the Voelund model.

control

list of arguments controlling constrained optimisation (zero as boundary), maximum number of iteration in the optimisation, relative tolerance in the optimisation, warnings issued during the optimisation.

Value

An object of class 'drc' with a few additional components.

Details

The function is a wrapper to drm, implementing the models described in Soerensen et al. (2007). See the paper for a discussion of the merits of the different models. Currently only the log-logistic models are available. Application of Box-Cox transformation is not yet available.

References

Soerensen, H. and Cedergreen, N. and Skovgaard, I. M. and Streibig, J. C. (2007) An isobole-based statistical model and test for synergism/antagonism in binary mixture toxicity experiments, Environmental and Ecological Statistics, 14, 383--397.