[R-sig-ME] VERY simple question about NESTING in experimental designs (for glm, lme, lmer, etc.)

[Andrew Dolman]

Hi Toby,

I believe that using unique labelling, as you have done, means that
you can specify a model in lmer like this:

 lmer(Birdlength ~ Country + (1|locnum) + (1|bnum))

and lmer will know that bnum is nested in locnum, rather than being crossed.

i.e.

 lmer(Birdlength ~ Country + (1|locnum) + (1|bnum))
and
 lmer(Birdlength ~ Country + (1|locnum/bnum))

are identical ONLY IF bnum is unique , otherwise these models would
specify crossed and nested structures respectively.


However,

If you only have 1 observation per bird then bnum should probably not
be part of your nesting structure.

 lmer(Birdlength ~ Country + (1|locnum))

would be the way I would go.

Country does not need to be in the random part of the model and indeed
probably shouldn't be because it only has two levels.


Andy.




andydolman at gmail.com



On 20 January 2011 15:23, Toby Marthews <toby.marthews at ouce.ox.ac.uk> wrote:
> Dear R-sig-mixed-models,
>
> This is probably a painfully simple question, but I can't seem to pin it down from any source. About NESTING.
>
> Imagine I carry out a nested experiment on birds. I measure the size (say in cm beak to tail) of birds from 6 different locations in Country A with 20 birds in each location. I then also repeat this experiment in Country B with the same replication. Crucially, because the work at each of the 12 locations was being carried out by a different collaborating group, they all numbered their individual birds simply 1-20. The data I eventually receive for my metastudy is something like:
>
> Country Location        Birdnum Birdlength
>   A    1       1       7.3
>   A    1       2       6.7
>   ...  ...     ...     ...
>   A    1       20      7.9
>   A    2       1       6.7
>   A    2       2       6.9
>   ...  ...     ...     ...
>   B    1       1       6.7
>   B    1       2       6.6
>
> The bird numbers and location numbers here are not unique across the experimental design (bird #1 at location 1 is not the same individual as bird #1 at location 2). Hearing what Prof Bates said about not using "implicit nesting" in 2005 (http://cran.r-project.org/doc/Rnews/Rnews_2005-1.pdf ), I construct new variables bnum=factor(paste(Country,"-",Location,"-",Birdnum,sep="")) which contains levels "A-1-1","A-1-2", ..., "B-1-2", etc., and locnum=factor(paste(Country,"-",Location,sep="")) which contains levels "A-1","A-1", ..., "B-1", etc. and that means I can use locnum and bnum instead of Location and Birdnum and I have a unique numbering system.
>
> Say I am interested in the differences between birds in countries A and B with location and birdnumber being random effects. I believe I should try to use a command like
>  lme(fixed=Birdlength~Country,random=~1|bnum)
>  or glm(Birdlength~Country)
>  or lmer(Birdlength~Country+(1|bnum))
> however I have been criticised on two counts for this by colleagues-who-shall-remain-nameless:
>
>    (1) This is a nested design so I should replace bnum with Country/locnum/bnum or Country/Location/Birdnum in both the lme and the lmer command. (I'm pretty sure I can just use bnum on its own because by knowing bnum I automatically know the corresponding country and location of the measurement so Country and Location are effectively redundant (surely?) however, if I'm right then that means that I will only ever need "/" if my nesting is somehow implicit (i.e. because I usually use paste in the way described, I should never have to use "/" even in nested experiments (which seems odd?))
>    (2) Because Country (my fixed predictor) is being used to calculate bnum, I am mixing fixed and random effects inappropriately. (I am less sure about this one: perhaps "random=~Country|bnum" would be more correct or something else?)
>
> Even with this very simple experimental setup, a number of possible alternatives have been suggested by my colleagues including
>  glm(Birdlength~Country+Country/locnum+Country/locnum/bnum)
>  lmer(Birdlength~Country+Country|locnum+Country|(locnum/bnum))
>  lmer(Birdlength~Country+Country|locnum+Country|(bnum))
>  lmer(Birdlength~Country+1|locnum+1|(locnum/bnum))
> with the result that I'm getting very confused!
>
> I'm pretty sure I'm making a meal of this little question so I'll stop there, but any comments would be very welcome!
>
> Best,
> Toby Marthews
>
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