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/
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--- 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.
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
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* http://www.ats.ucla.edu/stat/stata/
