-------- Original Message --------
Subject: Re: Two group CVA plot
Date: Wed, 3 Sep 2008 06:44:07 -0700 (PDT)
From: <[EMAIL PROTECTED]>
To: [email protected]
To respond to Rebecca's question below, in CVAGen, the CVA is carried
out using a Matlab function which computes the eigenvectors of the
matrix resulting from dividing the between-group variance-covariance
matrix by the estimated pooled within-group variance-covariance matrix.
So if there is only 2 groups, there will be only one meaningful CV axis
(1 eigenvector). However, the algorithms used to compute the
eigenvectors does return a set of orthogonal axes from the eigenvalue
decomposition. These higher ordered axes are orthogonal to the one
another, and to the meaningful axes.
The program is set up to allow you to plot any chosen axis against
another, so that in a case with 3 meaningful axes, you could plot axes 1
and 2, and 1 and 3, etc. The default plot setting in CVAGen shows axes
1 and 2.
In a case where there are only two groups, the program will plot scores
along the second CVA axes, but I would not advise trying to interpret
this axis, it is simply the second eigenvalue of the decomposition,
orthogonal to the first. In such cases, the distribution of scores
along the first axis indicates to some extent the effectiveness of the
discrimination, although I would urge use of cross-validation or
jackknife rates of specimen assignment as a better measure of
effectiveness. The plot of scores along the axis reflects more
information about the resubstitution rate of assignment, ie of specimens
to which the function was fitted.
Perhaps others can add comments about approaches to displaying the
results of two group discrimination functions. Plotting histograms
would be one option, but there may be better approaches.
-Dave
H. David Sheets, PhD
Dept of Physics, Canisius College
2001 Main St
Buffalo NY 14208
---- Original message ----
Date: Wed, 03 Sep 2008 09:07:20 -0400
From: morphmet <[EMAIL PROTECTED]>
Subject: Two group CVA plot
To: morphmet <[email protected]>
-------- Original Message --------
Subject: Two group CVA plot
Date: Tue, 2 Sep 2008 07:53:07 -0700 (PDT)
From: Rebeca <[EMAIL PROTECTED]>
To: <[email protected]>
Dear Morphometricians,
I have been using the program CVAGen6j from the IMP series (David
Sheets) to compare the shape differences of two groups of fish samples.
The program gave me a CVA scatterplot with two axes. I would like to use
this graph because it is more appealing than a histogram, but I don’t
know what is plotted on the second axis (y-axis), as I understand that
from a two group analysis you get only one canonical variate (the x-axis
in this case). Does anyone know how the plot is done?
Thanks a lot,
Rebeca
Rebeca P. Rodriguez Mendoza
e-mail: [EMAIL PROTECTED]
CSIC
Marine Research Institute – Fisheries
C/ Eduardo Cabello, 6
Tel. 986 23 19 30 ext. 254
Fax: 986 29 27 62
36 208 Vigo, Spain
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