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, [EMAIL PROTECTED] 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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