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RE: Dollodon status questioned

Liz -

Actually, running a PCA on three variables is a perfectly reasonable thing to 
do, especially if you want to, say, extract a size axis/variable based on those 
three variables and if you reasonably consider PCA more of a mathematical 
method than a statistical one. You can even do it on two variables, which then 
essentially is the basis for doing major axis line fitting (as opposed to 
reduced major axis or standard regression). So no statisticians should infarct 
over it.

That the PCA will give you, most probably, a PCI that is a huge size factor, 
especially for dinosaurs, especially for sauropods, is indeed true as well 
depending on the size distribution of the animals measured. There are some 
animals, however, where size does include taxonomically important data - bats, 
for example, but they tend to have an adult size rather than grow continuously 
and it is especially common when adult size is really constrained by, say, 
flying. Also, sometimes the taxonomically important variation gets caught up in 
that size data and it gets tricky to extract/interpret. So one would want to be 
retrospective about using it as such for sauropods but, depending on the sample 
size and size distribution, there could be some neat info there.


-----Original Message-----
From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf Of 
Denver Fowler
Sent: Friday, March 25, 2011 12:32 PM
Subject: Re: Dollodon status questioned

Forwarded for Liz Freedman 

On Fri, Mar 25, 2011 at 1:36 AM, Jaime Headden <qi_leong@hotmail.com> wrote:

>   I wonder if McDonald uses some more substantive reasoning (optimistically, 
> I 
>assume he does) than in the abstract -- which is, let's face it, an abstract. 
>haven't the paper, but will soon.

Not really - he runs a PCA on a mere 3 measurements for each specimen
(which would give any statistician a heart attack), and those 3
measurements are all known to vary with the size of the specimen
(i.e., likely ontogenetically). Axis 1 (80% of variance) is probably
based entirely on the size of the specimen, with no real taxonomic

-Liz Freedman