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1

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.

2

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 ?

3

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.

4

To me it seems meaningless to insert in a table sites with no species or

viceversa.

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?

5

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

federico@syspr03.disat.unimi.it

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