[R] Variance explained in mixed models

Simon Pickett S.Pickett at exeter.ac.uk
Fri Aug 31 15:42:52 CEST 2007


I realise this has come up before in various reincarnations but I couldnt
find the answer...

I wish to quote the "percentage variance explained" by each of three
components in my mixed model, one random effect and two fixed effects.

lmer(response~x1+x2+(1|random), data=data)

Using lmer I can get the variance explained by the random effect but not
the fixed effects obviously.

I was using a suggestion by someone that you can compare the variance
components of the model with with both fixed terms included, with another
model that has one of the fixed terms removed. When I did this the
Variance + residual variance did not add up to the same figure each time.

#first model with both fixed terms
          Variance StdDev
(Intercept) 1.074666 1.036661
Residual    1.136264 1.065957

#second model with one term ommitted
            Variance StdDev
(Intercept) 1.113713 1.055326
Residual    1.154527 1.074489

#third model with the other term ommitted
Variance StdDev
(Intercept) 1.069478 1.034156
Residual    1.145590 1.070322

Actually the second model with only one fixed effect is saying that the
percentage variance explained is higher than the one with both fixed
effects which is impossible right?

Any better ideas?

Thanks in advance, Simon.

Simon Pickett
PhD student
Centre For Ecology and Conservation
Tremough Campus
University of Exeter in Cornwall
Tel 01326371852


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