There is also an excellent section on what constitutes a random or fixed effect in Tom Hobbs and Mevin Hooten's "Bayesian Models: a Statistical Primer for Ecologists" using fecundity of spotted owls (adapted from Clark's work on the subject), and again using hypothetical sampling of aboveground biomass, as examples. Both examples are accompanied by clear and concise explanations of the implications for the underlying distributions and assumptions of the model one might seek to fit, and for the ecology informing the models.
Garrett Street Assistant Professor Wildlife, Fisheries, and Aquaculture Mississippi State University On May 17, 2016, at 4:34 PM, Brian Church <[email protected]<mailto:[email protected]>> wrote: There is a fairly detailed discussion of fixed vs. random effects on CrossValidated here: http://stats.stackexchange.com/questions/4700/what-is-the-difference-between-fixed-effect-random-effect-and-mixed-effect-mode Based on the discussion there, it seems like temperature, rainfall, and density could all be considered to be random effects for the following reasons: 1. You are unlikely to sample the entire populations for those variables. 2. They are not being controlled 3. They are likely continuous and distributed in some way (e.g., normal) rather than discrete values 4. You are unlikely to be interested in responses at a specific temperature, rainfall, and density; rather, it seems more interesting to understand effects relating to the underlying distributions of those variables. Those commenting in the CrossValidated forum cite a few sources, though they seem to be general/mathematical rather than ecology-specific. Hope that helps some. -Brian Church On Tue, May 17, 2016 at 11:12 AM, Gary Grossman <[email protected]<mailto:[email protected]>> wrote: I'm having a bit of difficulty getting a clear understanding of what should be considered a fixed vs. a random effect in a linear mixed model analysis of field data. Even the statisticians seem to say "it depends on who's defining it" or "sometimes the same treatment/variable can be either". Some examples may help, let's say I collected samples annually in three sites and wanted to test for the effect of daily rainfall, daily temperature, and density, on recruitment of individuals in the following year. Using the lmer function in R which of these would be fixed effects and which would be random? A reference or two would help. I really couldn't find much in a google search on field studies, but I didn't go to anything like zoological abstracts. TIA, g2 -- Gary D. Grossman, PhD Fellow, American Fisheries Soc. Professor of Animal Ecology Warnell School of Forestry & Natural Resources University of Georgia Athens, GA, USA 30602 Website - Science, Art (G. Grossman Fine Art) and Music www.garygrossman.net<http://www.garygrossman.net> Board of Editors - Animal Biodiversity and Conservation Editorial Board - Freshwater Biology Editorial Board - Ecology Freshwater Fish Hutson Gallery Provincetown, MA - www.hutsongallery.net/artists.html<http://www.hutsongallery.net/artists.html>
