On Fri, 20 Apr 2001, William Levine wrote:
> 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.
Here's my take on it... The ANCOVA model can be implemented with
sequential/hierarchical regression as you note below... however, ANCOVA
has at least two assumptions that your situation does not meet. First, it
assumes that assignment to treatment condition is random. Second, it
assumes that the measurement on the covariate is independent of
treatment. That is, the covariate should be measured before the treatment
is implemented. Thus, I believe that you should implement the
hierarchical regression... but I'm not certain what question you are
really answering...
I guess it is whether there is variabilty in memory for order that is
related to age, that is independent of variability in memory for
items... So, I would not call it an ANCOVA... You might also consider the
possibiltiy of interaction... That is, is the relationship between memory
for order and memory for items the same for younger and older
participants...
WBW
__________________________________________________________________________
William B. Ware, Professor and Chair Educational Psychology,
CB# 3500 Measurement, and Evaluation
University of North Carolina PHONE (919)-962-7848
Chapel Hill, NC 27599-3500 FAX: (919)-962-1533
http://www.unc.edu/~wbware/ EMAIL: [EMAIL PROTECTED]
__________________________________________________________________________
>
> 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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