Re: [R] 0-only-rows in correspondence analysis

[Stephane DRAY]

CA is based on relative composition and not on absolute value. So, empty 
site do not give any information. In a computational point of view, empty 
row or column do not allow to compute row / col weights. that is why most 
function do not work with empty row or column. dudi.coa (ade4 package) 
allows empty rows or columns. They are simply put at the centroid (and so 
have no weight in the analysis). CA do not take into acccount the 
information given by empty site !!! If you want that the information given 
by empty site are taken into account, you should prefer an other analysis 
(e.g. pca).

Sincerely,

At 10:09 22/10/2003, you wrote:
Hello,

I am looking for a workaround of "species empty" plots in a correspondence
analysis.
I tried to do a community structure analysis with a ca via decorana(), ca()
and CAIV(), but none of them allow 0 only-rows.
I have species (abs./pres) in columns and sites in rows

        sp1 sp1 sp3 ...
plot1   0    1     1
plot2   0    0    0
plot3   1    0     1
.....
but on some plots no species could be found, therefore only 0 appear. I know
that this isn't interesting for a community analysis but I am interested in
the contributions of variables to the factors for a further analysis with
env. variables where it is interesting that there are no species found.
i thought about adding a "dummy" species with only presence on all sites, 
that
would solve the 0-only-row problem but I am not sure if that would bias the
outcome of a ca seriously.

Or is there a subcommand to allow 0 rows in a ca which I haven't seen?

thnaks Martin

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