Message: 2
Date: Wed, 13 Apr 2011 11:18:24 -0400
From: <Solene.Tremblay-Gendron at UQAR.QC.CA>
To: <r-sig-mixed-models at r-project.org>
Subject: [R-sig-ME] Interaction with quadratic variable
Message-ID:
? ? ? ?<1AFF299063CAB341B3C55EA50E451DD487B8B0 at owadepot.bureautique.uqar.qc.ca>
Content-Type: text/plain
Hi,
I got some questions about assumption and interpretation of conditional logistic regression. It seems that two of my variables (let says N and O) have non-linear effects. In fact, their effects look parabolic, so I transform O and N into quadratics variables (now N+N2 and O+O^2) in my model. The variable O (now became O+O^2) is the most interesting variable in my model and I want to look for interaction of this variable with some others including variable N (now became N+N^2).
The questions are:
1. Is the transformation necessary (what does the transformation do to the variable/model) for the assumption and how do I interpret quadratics terms after (O+O^2)?
2. How do I put interaction between a quadratic and a simple variable (O^2:O:X, O^2:X + O:X, etc.)?
3. How do I put interaction between two quadratics variables in the model (O^2:N^2:O:N, O:N+ O^2:N + O:N^2: O^2:N^2, etc.)
4. How do I interpret these quadratics interactions?
Thanks,
Sol?ne Tremblay-Gendron