(no subject)

From: Federico Spinazzi (federico@SysPr03.disat.unimi.it)
Date: Tue Oct 07 1997 - 13:39:23 MET DST

I would like to make some observations on a species-centered PCA on a
site-species count table.
It could be possible that the first component is very often something like
a size component ?
I found a correlation beetwen the first component and 1/(Simpson's D)
and/or Shannon's H up to 0.7.

On frequencies tables, if they are espressed as %, every row will sum up
to 100, for example.
In such cases we have compositional data.
It does not seem appropriate to use PCA on such a table because of the
spurious correlation that could occur beetwen variables.
We can use some trasformation, instead (centered logratio or so on).
Do you agree ?

Sometimes it happens that a categorical scatterplot (Option|Elipses) 'does
not work' after a Reciprocal Scaling.
The module gives out the message 'no item in category ...'.
When does such a situation occur ?
I tried to undertand the matrix algebra behind Reciprocal Scaling but with
not a great success.

To me it seems meaningless to insert in a table sites with no species or
Infact I noticed that ADE often crashes when trying to perform PCA or COA
on such a table.
Otfen, but not always. What does it happens if some row or some columns
sums to zero?
How we can consider the row and column score in such situations ?
Are they correct or the fact that program finished the job was only a
realization of an improbable event?

Wich is the best method to rapresent units each of whom is a set of fish
on wich we count parassites?
Via MCA (how many fish have no parassites, how many fish...).

I would like to thank you all for the work on ADE.

Federico Spinazzi
Dipartimento di Scienze dell'Ambiemte e del territorio
Universita' di Milano

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