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st: RE: Categorical variable codings


From   "Maarten Buis" <M.Buis@fsw.vu.nl>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Categorical variable codings
Date   Thu, 19 Oct 2006 13:50:30 +0200

Christopher:
the -xi3- prefix (see: -findit xi3-) is a convenient way to add your
categorical variables using a variety of different types of contrasts 
and/or two or three way interactions. Which contrasts and interactions
to use is of course dependent on your knowledge of the problem, and 
the question you want to answer.
HTH,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology 
Vrije Universiteit Amsterdam 
Boelelaan 1081 
1081 HV Amsterdam 
The Netherlands

visiting adress:
Buitenveldertselaan 3 (Metropolitan), room Z434 

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

--- Silwood, Christopher J wrote:
I am trying various regression analyses on data obtained from the
study of 6 different cooking oils, control and 4 different material
treatments, all samples being sampled at 30 min, 60 min and 90 min
heating. I am wondering what a suitable set of categorical variable
codes to use for comparing, e.g. material/time or oil/material/time,
i.e. 5 X 3 or 6 X 5 X 3 at all levels and interactions, assuming with
the latter that "oil" can be included as an independent variable.


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