On Wed, 12 Nov 2003, Landis, R Matthew wrote: > Dear R-help folks, > > Can someone guide me to a source where I can learn more about the / operator > in model formulae? I found a reference to it in Venables and Ripley's MASS, > p. 142, where it says, in reference to ANCOVA: "Terms of the form a/x, > where a is a factor, are best thought of as 'separate regression models of > type 1 + x within the levels of a.'..."
See p.150 for a/b where b is a factor. > This seems very appropriate to my analysis, where I am doing an ANCOVA of > tree growth as a function of tree height (ht - continuous) separately for > three species (spp) and three light levels (lt). The problem is, I'm not > exactly sure how to interpret the results of this model specification, and I > can't find any other references to it despite doing searches in the help > pages, Google, Jonathon Baron's R site search, and other text books such as > Dalgaard or Crawley. > > I've fit the model as: growth ~ spp*lt / ht -1, and compared the results to > growth ~ spp*lt*ht. > > The coefficients are equivalent in both models, producing identical plots of > predicted values. but the terms that appear in the summary table differ, as > do the P-values, and I'm not sure how to interpret them. In the more > standard model (growth ~ spp*lt*ht), I am clear on the fact that each level > of the main effects is compared to the lowest level of that main effect > (when using contr.treatment). But the alternative model (growth ~spp*lt / > ht -1) contains all three levels of the main effect, spp. That is caused by the -1. The difference is between testing and crossing. In a / b the levels of b are unrelated for different levels of a, whereas in a*b they are the same levels. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help
