R-beta: formula() and model formulae

[Gregory R. Warnes]

On Wed, 7 May 1997, Bill Venables wrote:

> A term such as (a + b + x + y)^2 should be expanded out binomial
> fashion, coefficients stripped away and the remaining products
> treated as : products.  Then S copes with terms like a:a, a:b and
> a:x fine, even x:y is handled by having it generate a column of
> xy-products, as it should.
> 
> But a term such as x:x does not generate a column of x-squares,
> it is merely removed as it would be if it were a factor.  This is
> a complete anomaly, and one that I don't think would be hard or
> dangerous for R to rectify.  Indeed it would be very useful to
> generate a complete second degree regression in three variables
> using y ~ (1 + x1 + x2 + x3)^2. 

Amen.  Let it be!

>  > (I don't have any strong feeling about this, personally.  As
>  > long as R won't introduce those awful Helmert contrasts as
>  > default...)

ditto!

-------------------------------------------------------------------------------
    Gregory R. Warnes          | It is high time that the ideal of success
warnes at biostat.washington.edu  |  be replaced by the ideal of service.
                               |                       Albert Einstein
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