[R] Difference between prcomp and cmdscale

From: michael watson (IAH-C) <michael.watson_at_bbsrc.ac.uk>
Date: Thu, 14 Jun 2007 09:33:42 +0100


I'm looking for someone to explain the difference between these procedures. The function prcomp() does principal components anaylsis, and the function cmdscale() does classical multi-dimensional scaling (also called principal coordinates analysis).

My confusion stems from the fact that they give very similar results:

my.d <- matrix(rnorm(50), ncol=5)
rownames(my.d) <- paste("c", 1:10, sep="")
# prcomp

prc <- prcomp(my.d)
# cmdscale

mds <- cmdscale(dist(my.d))
cor(prc$x[,1], mds[,1]) # produces 1 or -1 cor(prc$x[,2], mds[,2]) # produces 1 or -1

Presumably, under the defaults for these commands in R, they carry out the same (or very similar) procedures?

Thanks
Mick

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