[R-sig-ME] Random slope models with nested random effects and multiple x variables

[ONKELINX, Thierry]

Dear Sam,

Models a and b are identical.

Model c has the problem that you fit two random intercepts for bird (one from the 1|bird/track term and one from the a|bird term

You might want the model lmer (behaviour ~ (a|bird)  + (1|bird:track))

Best regards,

Thierry

> -----Oorspronkelijk bericht-----
> Van: r-sig-mixed-models-bounces at r-project.org [mailto:r-sig-mixed-models-
> bounces at r-project.org] Namens Samantha Patrick
> Verzonden: donderdag 6 oktober 2011 12:20
> Aan: r-sig-mixed-models at r-project.org
> Onderwerp: [R-sig-ME] Random slope models with nested random effects and
> multiple x variables
> 
> Hi
> 
> I am fitting random intercept and slope models on some GPS tracking data.  I
> have data from 113 tracks from 31 individuals with about 400 behavioural
> observations per track.
> 
> I am interested in looking at how individuals change their behaviour in response
> to an environmental variable (a) but want to control for the non-independence
> of points from individual tracks.
> 
> So I have these possible lmer models :
> 
> 
> a)       lmer (behaviour ~ (1|bird/track) + (-1+a|bird))
> 
> b)      lmer (behaviour ~ (1|bird/track) + (0+a|bird))
> 
> c)       lmer (behaviour ~ (1|bird/track) + (a|bird))
> 
> 
> I have two questions:
> 
> 1)     Can you fit an intercept and slope with different random effect structures?
> In theory I think this is ok but I have not see it done.
> 
> 2)     I am interested in the differences in covariance structures in the three
> models.  I thought that model (a) set the covariance between the slope and
> intercept for bird to 0 but from looking at previous posts in the forum, it seems
> that model (b) may do this?  In which case does model (a) set the covariance to -
> 1?
> My understanding is that model (c) allows a symmetric variance-covariance
> matrix (which is what I want) but I am concerned that this model is fitting two
> intercepts for bird?
> 
> Ideally I want the intercept for bird to be allowed to covary with the slope for
> bird (I don't want to constrain it  to 0 or 1) but I am unsure if my problem is in the
> syntax or whether I don't fully understand how the model partitions the variance
> in nested random effects and that maybe the model I want to fit is not possible.
> 
> Any advice on creating the models would be much appreciated!
> 
> Many Thanks
> 
> Sam
> 
> Dr Samantha Patrick
> EU INTERREG Post Doc
> Davy 618
> Marine Biology & Ecology Research Centre University of Plymouth Plymouth
> PL4 8AA
> 
> T: 01752 586165
> M: 07740472719
> 
> 
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