[R-sig-ME] Fixed effects structure

Ned Dochtermann ned.dochtermann at gmail.com
Thu Dec 2 22:45:08 CET 2010

List members,

I have a question regarding the structure of fixed effects and I was hoping
to get a bit of feedback and hopefully direction to a few references. I
figured this topic should be okay here since "mixed" necessarily includes

A colleague of mine is working on a project where she is primarily
interested in the effects of an experimental treatment and the interaction
of the experimental treatment with another fixed factor. She is not,
however, interested in the main effect of this additional factor. 

I know that the conventional wisdom has been that you shouldn't include a
term in an interaction if it isn't also included as a main effect (based on
undergraduate stats mantra). My comment was that since the statistical
hypothesis is meant to represent the biological hypothesis, not including
the main effect *might* be okay. She has some power concerns and would
prefer to maximize her denominator degrees of freedom (yes it is actually a
mixed model; yes I'm aware of the problems with calculating the dfs).

Are there compelling statistical reasons against not including the main
effect? Are there good references discussing this (I know she's gone through
Sokal & Rohlf and a few other books) that would support either argument?

Cliff notes:
is: ~ trt + trt:grp
versus: ~ trt + grp + trt:grp

Thanks for any feed back,

Ned Dochtermann
Department of Biology
University of Nevada, Reno

ned.dochtermann at gmail.com

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