On 12 May 2008, at 17:09, Douglas Bates wrote:
I'm entering this discussion late so I may be discussing issues that
have already been addressed.
As I understand it, Federico, you began by describing a model for data
in which two factors have a fixed set of levels and one factor has an
extensible, or "random", set of levels and you wanted to fit a model
that you described as
y ~ effect1 * effect2 * effect3
The problem is that this specification is not complete.
My apologies for that, I thought that the above formula was the
shorthand for what I would call the 'full' model, i.e. the single
factors and the 2 and 3 ways interactions.
An
interaction of factors with fixed levels and a factor with random
levels can mean, in the lmer specification,
lmer(y ~ effect1 * effect2 + (1| effect3) + (1|
effect1:effect2:effect3), ...)
or
lmer(y ~ effect1 * effect2 + (effect1*effect2 | effect3), ...)
or other variations. When you specify a random effect or an random
interaction term you must, either explicitly or implicitly, specify
the form of the variance-covariance matrix associated with those
random effects.
I'll play around with this and see what I can get.
The "advantage" that other software may provide for you is that it
chooses the model for you but that, of course, means that you only
have the one choice.
I'm more than happy to stick to R, and to put more legwork into my
models
If you can describe how many variance components you think should be
estimated in your model and what they would represent then I think it
will be easier to describe how to fit the model.
I'll work on that. Incidentally, what/where is the most comprehensive
and up to date documentation for lme4? the pdfs coming with the
package? I suspect knowing which are the right docs will help a lot
in keeping me within the boundaries of civility and prevent me from
annoying anyone (which is not something I sent forth to do on purpose).
Best regards,
Federico
--
Federico C. F. Calboli
Department of Epidemiology and Public Health
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG
Tel +44 (0)20 75941602 Fax +44 (0)20 75943193
f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com
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