Joe asks about models for compositional data. We have an example in this paper on toxicology from 1996 (also briefly covered in the “nonlinear models” chapter in BDA). In the paper, it’s discussed in the last paragraph of p.1402 and continues on to p.1403.
Models for compositional data
There is an R package for compositional data – compositions . Description on CRAN has also link to developer pages with additional material.