There should be no problem with using a priori contrasts. No
adjustment of the p-value is required, UNLESS the contrasts are
non-orthogonal, which is often the case when more than two hypothesis
tests. If the contrasts are not orthogonal, some type of adjustment
(e.g. Bonferroni) is required, which will lower the power you would
otherwise gain with a planned contrast.
Casey
---------------------
Casey terHorst
Ecology & Evolution Program
Department of Biological Science
Florida State University
319 Stadium Drive
King Life Science Building
Tallahassee, FL 32306-4295
terho...@bio.fsu.edu
Quoting "Asaf Sadeh" <asaffi...@yahoo.com>:
I am planning a 2x2 factorial experiment whe
Dear list members,
I am planning a 2x2 factorial experiment where I record several response
variables to test several hypotheses.
Some of these hypotheses can be tested using the usual 2-way ANOVA.
That is, the
significance or non significance of the two main effects and their
interaction is expected to provide sufficient information to draw conclusions
regarding these hypotheses.
However, another hypothesis focuses on one of the four treatment
combinations, and would be best approached by comparing this focal treatment
with the other three. However, if I use the 2-way ANOVA, some
possible results
may be inconclusive, and require further "post hoc" analyses with
a reduced threshold for significance.
Since my a-priori hypothesis calls for 3 specific contrasts, it makes
statistical sense to me that I can skip the 2-way ANOVA and only
perform these 3
contrasts without reducing the significance threshold. The logic
behind this is
that these 3 planned contrasts replace the 3 component tests of the
2-way ANOVA
procedure (2 for the main effects and one for their interaction) that are
each always done without any change to the significance threshold.
Though I have found support for this approach on online statistics lectures
notes, I have never seen it taken in any published study, and I
wonder if it is
indeed kosher (and if it can be expected to pass reviewers).
I would highly appreciate input on this, and especially references
to back this
approach in case it is valid.
Thank you very much,
Asaf
