[R-sig-ME] Excluding random intercepts?
ethomas at stanford.edu
Fri Dec 7 15:47:36 CET 2012
kirsty, in addition to tom's comments, with which i agree, i wanted to ask if, in "covariance parameter estimate for the intercept term in the random slope model", you meant "variance of intercept". it seems that this is what you meant, but the correlation between intercept and slope might also be interesting if it is moderate (e.g., -.3 to -.6), depending on how you code 'time' (as tom has already said).
On Dec 6, 2012, at 1:43 PM, Kirsty E. B. Gurney wrote:
> Good afternoon;
> I have recently come across a *general question* about linear mixed models that I can't seem to find an answer for on my own. I am hoping that one of the braintrust on this list might be willing / able to help.
> Specifically, I am analyzing a dataset that describes pond temperatures for a series (n = 25 ponds, 436 measurements). Ponds were assigned to 1 of 4 treatments, and I want to find find out if temperature was affected by treatment.
> Pond-level inference, in general, is not of interest, so it seemed clear that POND should be included as a random effect. However, I have good reason to believe that the change in temperature across the study was also variable by POND, and a model that includes a random slope term has good support, as determined by AIC.
> However, as it turns out, the covariance parameter estimate for the intercept term in the random slope model is pretty small (i.e., I think most ponds start out at a pretty similar temperature).
> I am just wondering if it makes good statistical sense to run a random slope model without the random intercept term?
> If so, does anyone know how one would one specify this in the model structure?
> Many thanks in advance,
> Kirsty E. B. Gurney, Ph.D.
> Alaska Cooperative Fish and Wildlife Research Unit
> Institute of Arctic Biology
> 209 Irving 1 Building
> University of Alaska Fairbanks
> Fairbanks, AK 99775
> T: (907) 474 - 7738
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