[R--gR] Modelformulae
In a slightly longer perspective, I think the "CoCoCg"-models are far from being sufficient, so the language should preferably be extendable to express many other models, and their combinations, to achieve a long-term goal of "Programming with models". This may not be easy to do in a single step, but there is no reason to exclude anything from the language which can be given a clear and unambiguous meaning. If terms are included like log(X) or X*Y*Z, the CoCoCg-parser can just say that this model is not available as a CoCoCg-model. But some crisp conventions should be formulated in any case. Steffen ----- Original Message ----- From: "Poul Svante Eriksen" <svante at math.aau.dk> To: "Steffen Lauritzen" <steffen at stats.ox.ac.uk> Cc: "gRlist" <R-sig-gR at stat.math.ethz.ch> Sent: Friday, August 20, 2004 8:33 AM Subject: Re: [R--gR] Modelformulae
A few comments - just to clear my mind. Steffen Lauritzen wrote:
Dear gR-folks The Danish gR-gang have been talking about describing a model language
for
graphical models that 1) could specify at least chain graph models, based on the most general hierarchical mixed models as described in Lauritzen (1996) [my book], section 6.4, pages 199-216.
(More
general than MIM-models).
Are we limiting the discussion to the Conditionally Gaussian(CG) case with independent and identically distributed observations? That would make life much more easy and clearly rule out X*Y*Z between continuous variables. And I have no problem with interpreting "~" as "log f ~" as long as we are careful to include quadratic terms of continuos variables. But it also rules out things like ~Z+log(X) and is far from having the same flexibility as the BUGS syntax. We might allow ~Z*(Z+log(X))|X and thereby leaving CG, but sticking to gaussian error. If we want a more general BUGS/glim-like syntax, we need to consider, how to include information on link and errordistribution. And it would be nice to facilitate correlation across unit, e.g. by allowing ~X*B(X)|B(X), where B is the backshift operator. Regards, svante -- ---------------------------------------------------- Poul Svante Eriksen, office G1-113 Department of Mathematical Sciences Aalborg University Fredrik Bajers Vej 7G, DK-9220 Aalborg East, Denmark tel.: (+45) 96358868 ----------------------------------------------------