Re: [R] Basic Dummy Variable Creation

[Spencer Graves]

	  One must estimate 2 coefficients for a 3-level factor.  I therefore 
prefer to look for a plausible order between the 3 levels so I can code 
them as -1, 0, +1 and then estimate linear and quadratic coefficients. 
Then for interactions, I look first for interactions between the linear 
effect of this factor and others, especially if the data do not support 
reasonable estimation of both interaction terms.  This sounds to me to 
be pretty close to what your client was requesting.

hope this helps.
spencer graves
Rolf Turner wrote:
In response to a question from Francisco J. Bido, about how to create
dummy variables, Doug Bates and others essentially said ``Don't.''
Which is good advice, but ....
Recently I encountered a problem involving a linear model with a
three level factor (levels low, medium, and high) crossed with linear
and quadratic terms in a continuous variate.  The client wanted (for
some reason --- perhaps I should have discouraged him more
forcefully) to compare the full model with a model in which there
were linear and quadratic terms for the high level of the factor, but
only linear terms for the low and medium levels.
The only way I could see of specifying the reduced model was through
using dummy variables explicitly.  I.e. I could see no way of
specifying such a model in the standard general linear model syntax.
(The client was actually working in SAS, but the same considerations
apply whether one is speaking SAS or R/Splus, it seems to me.)
Did I miss something obvious (or even not-so-obvious)?

				cheers,

                                        Rolf Turner
                                        [EMAIL PROTECTED]
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