models with fixed effets nested in random effects
Hi Wayne, your question points to a broader problem with the specification of fixed and random effects. It is often true that fixed effects are aliased with elements of the underlying experimental design - such as in split plot designs - and then it makes sense to declare the effect as both fixed and random. (Actually I think that it makes more sense to create a new variable identical to the first and use one for fixed and the other for random, but that is neither here nor there.) I'm not sure whether this is true in your case. Anyway, I'm confused that you write "The treatment was applied: Location/Subsite/TREATMENT/year" and then in your model statement you have only three levels: FenceEnd/FEsection/MIT_UNMIT. Also, it seems that two treatment effects appear in the random statement. I can't reconcile that with your earlier description. Can you try to clarify these - perhaps by giving a more complete description of the design? Cheers, Andrew
On Fri, Mar 02, 2007 at 11:44:01AM -0700, Hallstrom, Wayne (Calgary) wrote:
I am having trouble defining a model that accounts for the data
structure but also allows the treatment effect to be estimated as a
fixed effect. I think I have it figured out but I would like to see what
some other opinions are regarding placement of a fixed effect in both
the random and fixed sections of a model formula.
There is 20 years of records at a variety of locations and subsites
within each location, with treatments for 1/2 of the years of records at
each location and subsite. The general structure to the data is:
Location/Subsite/year
The treatment was applied:
Location/Subsite/TREATMENT/year
The model format is:
u5 <- lmer(ung ~ FEsection + MIT_UNMIT +
(1|FenceEnd/FEsection/MIT_UNMIT), family=quasipoisson(link =
"log"))
Model output is:
> summary(u5)
Generalized linear mixed model fit using Laplace
Formula: ung ~ FEsection + MIT_UNMIT + (1 |
FenceEnd/FEsection/MIT_UNMIT)
Family: quasipoisson(log link)
AIC BIC logLik deviance
1661 1733 -816.7 1633
Random effects:
Groups Name Variance Std.Dev.
MIT_UNMIT:(FEsection:FenceEnd) (Intercept) 0.268713 0.51838
FEsection:FenceEnd (Intercept) 0.066643 0.25815
FenceEnd (Intercept) 0.213373 0.46192
Residual 1.371364 1.17105
number of obs: 1230, groups: MIT_UNMIT:(FEsection:FenceEnd), 93;
FEsection:FenceEnd, 58; FenceEnd, 7
Fixed effects:
Estimate Std. Error t value
(Intercept) -1.808774 0.340329 -5.315
FEsection2 -0.148425 0.349608 -0.425
FEsection3 -0.066525 0.344542 -0.193
FEsection4 -0.021216 0.346025 -0.061
FEsection5 0.470120 0.333193 1.411
FEsection6 0.459920 0.329556 1.396
FEsection7 0.455736 0.381993 1.193
FEsection8 0.006034 0.399353 0.015
FEsection9 0.423509 0.383930 1.103
FEsection10 0.062848 0.393176 0.160
MIT_UNMITunmit 1.572822 0.188382 8.349
This seems fine to me, there are the propoer number of groups in the
data structure. The problem I have yet to wrap my head around is the
fact that the fixed effects are still in the random effects category as
well. Is that a problem? Should I be adding all terms with fixed effect
variable, including interaction terms, into the 'fixed effects' part of
the model equation? Since I am new to this LMER routine it is a bit
confusing to write.
Wayne Hallstrom
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