[R-sig-ME] model specification for continuous environmental vars

[Tim Howard]

>Date: Fri, 13 Dec 2013 11:14:39 +0100
>From: Hans Ekbrand <hans.ekbrand at gmail.com>
>To: r-sig-mixed-models at r-project.org
>Subject: Re: [R-sig-ME] model specification for continuous
>environmental vars
>Message-ID: <20131213101439.GC15352 at pc245.socio.gu.se>
>Content-Type: text/plain; charset=us-ascii
>
>On Thu, Dec 12, 2013 at 01:39:04PM -0500, Tim Howard wrote:
>> List members - 
>> I am learning a lot, quickly, but still have a way to go. I would 
>> greatly appreciate some help with model specification in glmer.
>> I can't find a good example that parallels what I've got.
>> 
>> My dataset consists of spatially-balanced random samples of rare 
>> plants within alpine summits. There were two sampling bouts (yr1 and yr2)
>> with yr2 collected 6 years after yr1. A new set of random plots were 
>> collected at each bout (e.g. new estimate of the population, not repeated 
>> measures). I would like to test the difference in plant density from yr1 to yr2, overall. 
>
>If you only want to test if there is a difference in plant density
>between yr1 and yr2, then I don't think you should include the
>environmental variables, since difference in the outcome that relates
>to changes in the environmental variables between yr1 and yr2 would be
>attributed to the enviromental variables and "hide" the actual
>differences in outcome between yr1 and yr2.
>
>mod <- glmer(count ~ samp + (1|summit), data = dat, family="poisson")
>
>would be more appropriate, I think.
>
>Inclusions of the envirmental variables should only be done if you
>want to explain differences between yr1 and yr2, not for estimating
>their size.
These are good points. My reasoning was that I need to control for these
variables somehow. What if I have higher densities in yr2 but it is completely 
due to sampling -- that my plots happened to be at elevations where there are
higher densities? I want to remove the effect of differences in elevation when 
testing for the differences between yr1 and yr2. 
 
Thank you for the feedback!
Best, 
Tim