[R] units of a biplot

Prof Brian Ripley ripley at stats.ox.ac.uk
Mon May 9 12:54:00 CEST 2005

First, I think you are talking about the princomp method of biplot, and 
not a general biplot.

There are many different biplots for PCA, some incorrectly implemented. 
The derivation is in MASS4 pp. 310-1, as well as the interpretation.  You 
need to understand what a biplot is to understand the units, which depend 
on the value of 'scale' and 'pc.biplot'.  See the note on the help page 
for biplot.princomp.

On Mon, 9 May 2005, Mathias Ditzen wrote:

> I am afraid this might be a stupid question, but after scanning a little bit 
> through the help archive and the internet, I didn't come up with an answer 
> myself.
> I have done a principle component analysis on a data set, which yields the 
> mean response of 14 olfactory neurons to 16 different odors. When plotting 
> the result in a biplot, it nicely shows me the odors in different positions 
> in the principle component space. Additionally, the neurons are shown as 
> vectors within this plot. My question now is: what are the units of the axes? 
> When doing the principle component in IDL, the relative positions of the 
> odors are the same, but units on the axes span a much wider range. 
> Additionally I couldn't figure out what the units in the variable space 
> (right and upper border in the biplot) do represent.

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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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