[R-sig-eco] PCA - omission of attributes in percentage data

[Roy Sanderson]

Dear list

I've had a query from a referee arguing that in PCA and other multivariate methods, one attribute (typically spp) should be omitted when the sum of all attributes in each sample is 100%.  This might make sense if you only had two or three spp (not that you'd be doing a PCA with so few), but I'm less clear on why it is needed with larger numbers of spp.

Many thanks
Roy

Roy Sanderson
School of Biology
Ridley Building
Newcastle University
Newcastle upon Tyne NE1 7RU
roy.sanderson at ncl.ac.uk
Tel: 0191 222 3044