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Subject: Sexual dimorphism: Procrustes ANOVA vs DFA
Date: Wed, 3 Aug 2011 05:38:23 -0400
From: Louis Boell <[email protected]>
To: <[email protected]>
Dear all,
I would like to ask for advice with the following problem:
in a study of mouse mandible shape using Procrustes coordinates, looking
at differences between inbred strains, I want to test for sexual
dimorphism (with sample sizes of about 8 males and 8 females each for
each strain and 14 landmarks).
I did two things:
1)to see whether there are shape differences between males and females
within strains, I used discriminant function analysis between females
and males for each strain separately. I found no significant differences
at all.
2)I performed Procrustes ANOVA on pairs of strains with sex and strain
as main effects. Depending on the combination of strains, I usually get
a significant strain effect, and either no significant sex effect, but a
significant strain x sex interaction, or a significant sex effect, but
no significant interaction.
My problem consists of my limited understanding of ANOVA and hence
limited ability to interpret the results. I have consulted the
literature, but textbooks are either cryptic to me or too superficial,
and they do not cover the answers to my questions:
1)Most often ANOVA is used in this context, and reviewers want me to use
ANOVA. Why is ANOVA “better” than DFA in this context?
2)Why do I get significant sex effects or sex x strain interactions with
ANOVA, while I find no significant sexual dimorphisms with DFA?
3)Why do I get, in almost all cases, either a significant sex effect, or
a significant sex x strain interaction, but almost never both sex effect
and interaction significant, and what does each result tell me about my
data?
I would greatly appreciate your help.
Best wishes,
Louis Boell
Louis Boell
Louis Boell
MPI für Evolutionsbiologie, Plön
August-Thienemannstr.2
D-24306 Plön
Tel.: 0049 4522 763 280
