On 12 May 2008, at 10:05, Ken Beath wrote:
There is only one random effect, so where does the crossing come
from ? The fixed effects vary across blocks, but they are fixed so
are just covariates. For this type of data the usual model in lme4
is y~fixed1+fixed2+1|group and for lme split into fixed and random
parts.
First off, whoa, an helpful reply! thanks for that, I hope I won't
sound sarcastic or aggressive because I do not mean to be either.
Regarding your comment, the experiment was replicated three times, in
3 different months. I would argue that for the fixed effects to be
meaningful, they must have an effect over an above the effect:month
interaction (because each fixed effect, and their interaction, might
vary between each replicate). I would then argue I need to calculate
1) fixed.effect1:random.effect
2) fixed.effect2:random.effect
3) fixed.effect1:fixed.effect2:random.effect
to test if fixed.effect1 is meaningful (and use 1) as the error); if
fixed.effect2 has is meaningful (and use 2) as the error);
fixed.effect1:fixed.effect2 is meaningful (and use 3) as the error).
I'm happy to be correct if I am wrong here.
The problems seems to be that you want lme to work in the same way
as an ANOVA table and it doesn't. The secret with lme and lme4 is
to think about the structure of the data and describe with an
equation. Then each term in the equation corresponds to part of
the model definition in R.
I'll try to do that.
Once I have sorted how to specify such trivial model I'll face the
horror of the nesting, in any case I attach a toy dataset I
created especially to test how to specify the correct model (silly
me).
I'm a bit lost with your data file, it has 4 covariates, which is
more than enough for 2 fixed effects, assuming block is the
grouping and y the outcome.
In the data file, 'selection' and 'males' are fixed effects, and
'month' is the effect I am using for the model we are discussing
here. The y was generatde with runif() just to have something, I'm
not expecting any intersting result, just to understand how to fit
the right model.
In the dataset 'line' is nested within 'selection' and 'block' is
nested within 'month'. That's the nesting I will have to take into
account once I get the more straightforward (sic!) model we're
discussing right.
Best,
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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