> On 17 Jan 2016, at 19:34 , Charles C. Berry <ccbe...@ucsd.edu> wrote: > > > IIRC, there are some heuristics involved harking back to the White Book. I > recall there have been discussions of whether and how this could be fixed > before on this list and or R-devel, but I cannot seem to lay my browser on > them right now. >
And IIRC: yup, and one of the issues is that (a) some rules work left-to-right (b) the logic is oblivious to the factor/vector distinction For factors a,b,c, what happens for ~a:b + b:c is that a:b gets the full term expansion since the marginals a and b are not in the model but since b is part of the fully expanded a:b, b:c gets the reduced form expansion as it would in ~b + b:c (the c-within-b thing). Swapping the terms gives you a different result, but at least it is the same model in the sense that the columns span the same subspace. If a and b are vectors, and c is a factor, you get the same logic: expand a:b fully, then treat b:c as in b + b:c. Unfortunately, a:b is just the product of a and b, whether or not it is fully expanded, so it doesn't really make sense to proceed as if b is contained in a preceding term. So the net result is that you end up with one column less than you probably wanted. -- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Office: A 4.23 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
