[R] A small error in mca ?

[Prof Brian Ripley]

> Date: Tue, 11 Jul 2000 14:03:58 +0200
> From: Emmanuel Charpentier <emmanuel.charpentier at sap.ap-hop-paris.fr>
> 
> Dear list,
> 
> Working the example in Stats complements to V&R 3rd ed., I found this :

Please do not send such comments to the R list before consulting the authors,
who might well not even be readers of R-help.

> > library(MASS)
> > library(mva)
> > data(farms)
> > plot(mca(farms,abbrev=TRUE),cex=rep(0.7,2))
> # ... Works OK
> # Sheer curiosity ...
> > plot(mca(farms,abbrev=TRUE,nf=4),cex=rep(0.7,2))
> Error in rep(p * X.svd$d[sec], c(n, n)) : invalid number of copies in "rep"
> 
> A bit of exploration in the MASS source shows (12th line of the mca function
> body) :
> 
>   fs <- X.svd$u[, sec]/rep(p*X.svd$d[sec], c(n,n))
> 
> Shouldn't that be 
> 
>   fs <- X.svd$u[, sec]/rep(p*X.svd$d[sec], rep(n,nf))
> 
> that gives the expected result ?

Probably, depending on who is doing the expecting.  If you look at the
help file you will see that nf=4 is not expected to be useful.

> Oh, BTW : Is mca identical or analogous to Benzecri's "analyse des
> correspondances" (analysis of a chi-squared distance table), or is this yet
> another analysis ?

The document you are reading (skimming?) says in the very first paragraph of
that section:

   Multiple correspondence analysis (MCA) is (confusingly!) a method for
   visualizing the joint properties of $p \ge 2$ categorical variables
   that does \emph{not} reduce to correspondence analysis (CA) for $p =2$,
   although the methods are closely related \citep[see, for
   example,][{}\S10.2]{Gower.Hand.96}.

"l'analyse des correspondances" translates as correspondence analysis,
as described in the main text, and you could try looking in the reference,
too, for further elucidation.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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