Here is a statistical issue that I have been pondering for a few weeks now,
and I am hoping someone can help set me straight.
A study was conducted to assess whether there were age differences in memory
for order independent of memory for items. Two preexisting groups (younger
and older adults - let's call this variable A) were tested for memory for
order information (Y). These groups were also tested for item memory (X).
Two ways of analyzing these data came to mind. One was to perform an ANCOVA
treating X as a covariate. But the two groups differ with respect to X,
which would make interpretation of the ANCOVA difficult. Thus, an ANCOVA did
not seem like the correct analysis.
A second analysis option (suggested by a friend) is to perform a sequential
regression, entering X first and A second to
test if there is significant leftover variance explained by A.
This second option sounds to me like the same thing as the first option. In
an ANCOVA, variance in Y that is predictable by X is removed from the total
variance, and then variance due to A (adjusted) is tested against variance
due to S/A (adjusted). In
the sequential regression, variance in the Y that is predictable by X is
removed from the total variance, and then the leftover variance in Y is
regressed on A. Aren't these two analyses identical? If not, what is it that
differs? Finally, does anyone have any suggestions?
Many thanks!
--
William Levine
Department of Psychology
University of North Carolina-Chapel Hill
Chapel Hill, NC 27599-3270
[EMAIL PROTECTED]
http://www.unc.edu/~whlevine
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