Re: interactions

[John Darrington]

On Mon, Apr 16, 2007 at 01:53:54PM -0400, Jason Stover wrote:
     On Mon, Apr 16, 2007 at 11:10:43AM +0800, John Darrington wrote:
     > If we were to follow approach 2, am I right in thinking that the 
     > 'interaction' data structure could be as large as the number of 
     > cases in the casefile?
     
     No. It would have either a hash of possible values (all unique), or a
     small function to get back and forth between a union value and a
     binary vector.

So, given an interaction involving N variables, from a datafile with M
observations, what is the upper bound on the size of this hash ?  
     
     > On the other hand, approach 1 sounds attractive, but there are things
     > that need to be considered:
     > 
     >  a) They'd have to be a special class of variable, which would not
     >  normally be displayed, written to system files etc.  So a new 
     >  enum dict_class  entry in variable.h  would be required. 
     > 
     >  b) I'm not sure how  existing code would deal with these
     >  'invisible' variables.  For example many procedures might iterate 
     >   through all the variables.  So dict_get_var_cnt might have to
     >   take a parameter so that we'd know if we were interested in
     >  'interaction' variables or not.
     
     These statements make me think approach 2 is the way, especially
     your comment b) above. 

That's my feeling too, but I'm not familiar enough with the details to
be sure.  

What does Ben think?

J'
     

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