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Re: interactions

From: Jason Stover
Subject: Re: interactions
Date: Wed, 2 May 2007 10:23:10 -0400
User-agent: Mutt/1.5.10i

On Wed, May 02, 2007 at 09:39:34AM +0800, John Darrington wrote:
> Having thought about this some more and tried a few experiments, I'm
> in favour of approach 2.
> We need not duplicate category.[ch] rather we can generalise it, so
> that the new structure can do the work of both. 

Okay. I had made some progress using approach 2, but have been sidetracked
by final exams. I'll get back to it next week.


> J'
> On Sun, Apr 15, 2007 at 03:06:17PM -0400, Jason Stover wrote:
>      To have a glm procedure, pspp needs a data structure to handle
>      interactions. An interaction can be thought of as another variable
>      which is a function of two or more variables, usually categorical,
>      like this:
>           Variable 1          Variable 2        Interaction
>               A                  B                 AB
>        E                 B                 EB
>        A                 C                 AC
>        E                 C                 EC
>      ...etc. The interaction term could be created in one of two ways:
>      Either 1) create a new variable in the dictionary that corresponds to
>      the interaction, or 2) create a new 'interaction' data structure
>      that contains all necessary mappings between existing variables and
>      the value of the interaction.
>      Approach 1 would add a variable to the dictionary, but would not
>      create any more observations in the data set. It would make coding any
>      procedures that use interactions easier than approach 2, because doing
>      so would mean the procedure doesn't need to know about much special
>      code to handle interactions. It would also prevent the need for having
>      any more obscure string-values-to-binary-vector code like that in
>      category.[ch]. Approach 1 would still require the creation of some
>      code to create the interaction, though it may not require the creation
>      of a specialized "interaction" data structure to be available for use
>      by all procedures.
>      Approach 2 doesn't require adding anything to the dictionary, but it
>      does mean that any procedures that need to use interactions would have
>      to create those interactions themselves. These interactions would
>      therefore be lost after the procedure exits, meaning that any other
>      procedure that needs interactions would have to recreate
>      them. Approach 2 also means writing more code that partly duplicates
>      the code already in category.[ch].
>      I favor approach number 1, but before I fiddle with the
>      dictionary, I thought I should ask.
> -- 
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