[R-sig-eco] mixed model for repeat obs
CL Pressland
Kate.Pressland at bristol.ac.uk
Tue Mar 24 16:35:27 CET 2009
I have a data set that is unbalanced and consists of:
67 SITEs measured over several YEARs every WEEK for butterflies (LEPS per
m). I'm interested in the MANagement code assigned to each site, but I have
also data on TEMPerature, average SUN and WIND. My guess is that a linear
mixed model would be most appropriate and have constructed this code:
model1<-lme(LEPS~MAN,random=~YEAR/WEEK|SITE)
The output gives me:
--------------------------------------------------------------------
Linear mixed-effects model fit by REML
Data: NULL
AIC BIC logLik
-37631.24 -37566.48 18824.62
Random effects:
Formula: ~YEAR/WEEK| SITE
Structure: General positive-definite, Log-Cholesky parametrization
StdDev Corr
(Intercept) 5.875102e-03 (Intr) YEAR
YEAR 1.392439e-06 -0.164
YEAR:WEEK 5.068196e-07 0.531 0.301
Residual 3.532589e-02
Fixed effects: LEPS ~ MAN
Value Std.Error DF t-value p-value
(Intercept) 0.009866718 0.001428957 9793 6.904841 0.00
MAN 0.000028304 0.001127429 65 0.025105 0.98
Correlation:
(Intr)
MAN -0.685
Standardized Within-Group Residuals:
Min Q1 Med Q3 Max
-2.70566579 -0.40089121 -0.18073723 0.05900735 19.16411466
Number of Observations: 9860
Number of Groups: 67
--------------------------------------------------------------------
This output confuses me greatly! I figure that this clearly means that
management has no effect on butterflies but how can I figure out what
effect SITE, YEAR and WEEK have on the data? Would I have to also include
them in the fixed effects side of the formula (I'm unsure if this is
allowed)? Also, how could I include my weather variables? Would they just
be placed on the fixed effect side of the formula? If they are correlated
(I expect the weather variables are) would I have to place them in an
interaction rather than separately?
Any help that would be given would be gratefully received!
Kate
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