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Treat nested effects also as random effects?

philbirget

Community Trekker

Joined:

Oct 29, 2011

I was catching birds in 2 different areas, each containing 4 sites. Obviously,a site can only be in one area or the other, meaning that the site is nested within  area. My effect of interest is the area and not the site. Will I need to treat the nested effect also  as a random effect?

i.e will the model be

Area

Site(Area) 

or

Area

Site(Area)&Random

I would be great if somebody could hel me out of that situation.

All the best

Phil

2 REPLIES
stevedenham

Community Trekker

Joined:

Jun 23, 2011

This is much more of thought question.  The inference space between the two expressions is different.  The first infers to the areas and sites only--the inference space is only those areas and sites sampled, answering the question "Is there a difference between these sites, and only these sites?"  The latter refers to a broader inference space--that the sites within each area are somehow a random sample of all possible sites, so that you are answering the question, "Is there a difference between these two areas, averaged over all possible sites within an area?"  So it comes down to which of these questions is closer to the research question at hand.  A good reference for this would be Littel et. al., SAS for Mixed Models, or the online SAS documentation--search on "inference space".

Steve Denham

philbirget

Community Trekker

Joined:

Oct 29, 2011

O yes, I see. Thank you Steve that's very helpful.

Phil