Re: R-alpha: batch mode and interactions

Bill Venables (
Wed, 5 Jun 1996 09:03:47 +0930

Date: Wed, 5 Jun 1996 09:03:47 +0930
Message-Id: <>
From: Bill Venables <>
To: Ross Ihaka <>
Subject: Re: R-alpha: batch mode and interactions
In-Reply-To: <>

Ross Ihaka writes:

 > The problem is the lack of a type check for integer vs real
 > representations.  I will fix this.
 > I am currently giving the modeling code a going over.  I hope that
 > this will fix many/some of the problems.  In particular given factors "a"
 > and "b" with levels "A", "B" and "H", "M", "L", the corresponding
 > coefficients (with treatment contrasts) will be
 > 	aA aB bH bM bL aA.bH aB.bH aA.bM aB.bM aA.bL aB.bL
 > with the coefficients aA, bH and interactions containing these strings
 > dropped to ensure a full-rank design matrix.  This modulo the "."
 > seems to be what S does.

Rather than use a "." for interactions would you consider using a
":" to reflect the (low level) operator that creates them?

Of course even this does not go far enough.  There are two types of
"interaction" between factors, the "A:B" type and the "B %in% A"
type which are equivalent in abstract terms to "B + A:B".  I think
labelling is potentially very important in this area but doing it
justice is going to generate labels far too long.  You can't win.


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