On Fri, 8 Feb 2002, Thomas Souers wrote:
>
> 2) Secondly, are contrasts used primarily as planned comparisons? If so, why?
>
I would second those who've already indicated that planned comparisons are
superior in answering theoretical questions and add a couple of comments:
1) an omnibus test followed by pairwise comparisons cannot clearly answer
theoretical questions involving more than two groups. Trend analysis is
one example where planned comparisons can give a relatively unambigious
answer (is there a linear, quadratic, etc trend?) where pairwise tests
leave the research trying to interpret the substantive meaning of a
particular pattern of pairwise differences.
2) planned comparisons require that the researcher think through the
theoretical implications of their research efforts prior to collecting
data. It is too common for folks to gather some data appropriate for an
ANOVA, without thinking through the theoretical implications of
their possible results, analyze it with an omnibus test (Ho: all the means
the same) and rely on post-hoc pairwise comparisons to understand the
theoretical meaning of their findings. In a multi-group design if you
cannot think of at least one meaningful contrast code prior to collecting
the data, you haven't really thought through your research.
3) your power is better. It is well known that when you toss multiple
potential predictors into a multiple regression equation you run the risk
of "washing out" the effect of a single good predictor by combining it
with one or more bad predictors. ANOVA is a special case of multiple
regression where each df in the between subjects line represents a
predictor (contrast code). By combining two or more contrast codes into a
single omnibus test you reduce your ability to detect meaningful
differences amongst the collection of non-differences.
Hope this helps.
Michael
*******************************************************************
Michael M. Granaas
Associate Professor [EMAIL PROTECTED]
Department of Psychology
University of South Dakota Phone: (605) 677-5295
Vermillion, SD 57069 FAX: (605) 677-6604
*******************************************************************
All views expressed are those of the author and do not necessarily
reflect those of the University of South Dakota, or the South
Dakota Board of Regents.
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