[R-sig-ME] why does my inter-annual SD = 0?

[Douglas Bates]

Can you tell us how many levels there are of region, site, zone and
year?  If the data are collected in only a few years it is difficult
to estimate a variance component and the ML and REML  criteria drive
the estimates to zero.  You may be overmodeling the data but it is
difficult to say without access to the data or at least a feeling for
how many observations and in what configuration they are.

On Mon, Oct 11, 2010 at 10:32 AM, Scott Bennett <sbennett at ceab.csic.es> wrote:
>
>
> Hi,
>
> I am applying a mixed model to calculate the variance components of different factors in our seagrass data. The model i was using looks something like:
>
> POMI14_vc <- lmer(POMI_14 ~ Depth + surveyor + (1|region/site/zone) + (1|year),
>  data = P_oceanica)
>
> When I apply this model, however, year comes out with SD = 0. Year, in this data set signifies inter-annual variation (in the health status of seagrass meadows), of which there is a considerable amount. That makes me believe that there is is a feature of the model which is 'absorbing' the inter-annual variation.
>
> Can you suggest why this may be occuring? What modifiations could i use to fix this?
>
> kind regards
>
> Scott Bennett
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>
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