[R-sig-ME] lmer model specification for nested random effects

Ben Bolker bbolker at gmail.com
Fri Dec 10 23:24:42 CET 2010

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On 10-12-09 11:40 AM, Douglas Bates wrote:
> On Thu, Dec 9, 2010 at 10:22 AM, Gustavo Betini <betinig at uoguelph.ca> wrote:
>> Hi, all,
>> I have a dataset with several individuals from different sites measured
>> several times. There are 7 different sites and around 10 individuals
>> from each site. My question is:
>> Is this what Douglas Bates call "implicit nesting"? If so, the recommendation is to create a new variable
> We would need to see the output of
> str(data)
> to decide.  Assuming that there are around 70 individuals in total,
> the question is whether the id factor has about 70 levels or about 10
> levels.  In the second case id is only meaningful within a site and to
> produce unique levels for each individual it is necessary to consider
> the site:id interaction.
> But that is not the way that most researchers would organize the data.
>  The sensible thing to do is to assign a unique level to each
> individual in which case (1|id) is a suitable specification.
>> data<- within(data, {siteid<- factor(site:id)}

  I agree that it's sensible and works well with lme4, but I don't know
about "how most researchers would organize the data" -- depends very
much on the researcher/field.

  Ben Bolker

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