----- Forwarded message from Antigoni
Kaliontzopoulou <[email protected]> -----
Date: Sat, 9 Mar
2013 08:17:53 -0500
From: Antigoni Kaliontzopoulou
<[email protected]>
Reply-To: Antigoni Kaliontzopoulou
<[email protected]>
Subject: Re: Compare Vector Directions?
To: [email protected]
Dear
Marlijn,
if I understand correctly what you are trying to do, there are
several points here:
Why is a 30 degrees angle showing a significant P
value (0.0001), but
a 150 degrees angle a P value of 1 ? I thought 150 degree angle is
actually like 30 degrees, but then with the vectors having opposite
signs?
Whether you get a significant result or not when comparing
vector directions highly depends a) on the number of observations examined for
each comparison and b) on the dispersion of these observations around the
vectors. As such, you cannot extrapolate numerical patterns from one comparison
to the other. You need to trust the p-values you obtain.
So that
vectors with a 150 degree angle between them would still be
more similar to each other (but with opposite signs) than vectors that
have an angle of 90 degrees between them?
Same as above, due to
sampling issues etc, your analysis seems to indicate that in one case (factor A)
you cannot statistically support a difference between the examined vectors
(vector of shape-CS and vector of shape-factorA), while in the other (factor B)
you can.
In my opinion, the relation between Factor B and shape is
very similar
to centroid size effects, as is Factor A, although in opposite
direction.
All other factors have a different relation to shape.
Hum... What I
see from the results you attached is that factors A and E have a similar effect
as size (vector directions are not distinguishable), while factors B, C and D do
not have the same effect as size. However, from these results you cannot say if
different factors have the same effect BETWEEN THEM. For that, you would need to
run the same kind of analysis, but comparing factor A to B, B to C and so
on.
Also, I am not sure what other analyses you have done until now, but
maybe starting from a more general framework (like an ANCOVA-type analyses to
examine interactions between factors and size) would be useful to get a first,
more general image. Finally, do not forget that vectors also have a magnitude,
other than a direction, which may (or may not) be interesting for your
study.
A methodological suggestion would be to also run the same analyses
using the tools included in the geomorph R library (trajectory.analisis
function), although I assume you should obtain the same results.
I hope
this helps
Antigoni
----- Forwarded message from Marlijn Noback <[email protected]> -----
Date: Thu, 7 Mar 2013 05:52:18 -0500
From: Marlijn Noback <[email protected]>
Reply-To: Marlijn Noback <[email protected]>
Subject: Compare Vector Directions?
To: [email protected]
Dear all,
With enthusiasm I have tried the feature of vector direction
comparison within MorphoJ. But there is something I do not understand.
I sincerly hope you can help me with this issue. I am new to this
analysis, so I am not sure how to interpret my data.
I am trying to see if some of the environmental factors I am analyzing
have similar effects on cranial shape as do changes in cranial size.
I compared the following vectors:
Regression of shape on factor A, B, C, D and E.
Regression of shape on centroid size.
And it gave me the following results:
Angles (in degrees)
CentSize1
Fac.A 148.621
Fac.B 32.194
Fac.C 64.088
Fac.D 76.127
Fac.E 130.435
P-values (parametric)
CentSize1
Fac.A 1.00000
Fac.B <.00001
Fac.C 0.00433
Fac.D 0.08267
Fac.E 0.99999
Why is a 30 degrees angle showing a significant P value (0.0001), but
a 150 degrees angle a P value of 1 ? I thought 150 degree angle is
actually like 30 degrees, but then with the vectors having opposite
signs?
So that vectors with a 150 degree angle between them would still be
more similar to each other (but with opposite signs) than vectors that
have an angle of 90 degrees between them?
In my opinion, the relation between Factor B and shape is very similar
to centroid size effects, as is Factor A, although in opposite
direction.
All other factors have a different relation to shape.
Is this the correct interpretation?
I would very much appreciate your help in this.
With kind regards,
Marlijn Noback
marlijn.noback ( at ) ifu.uni-tuebingen.de
----- End forwarded message -----
--
Antigoni Kaliontzopoulou
CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos
Campus Agrário de Vairão, 4485-661 Vairão
PORTUGAL
Department of Ecology, Evolution, and Organismal Biology
Iowa State University, Ames,
Iowa 50011, USA
tel: +351 91 3086188
mail to: [email protected]
[email protected]
----- End
forwarded message -----
