Hi
On 25 Apr 2000, Simon, Steve, PhD wrote:
> I'm helping out someone taking a Statistics class and her instructor is
> drawing a distinction between "part correlation" and "partial correlation".
> I had never heard of the term "part correlation" before.
As others have pointed out, part is synonym for semi-partial.
> As best I understand it, If you have three variables, A, B, C, then you can
> compute residuals for A (call it A-A') for a regression model using A as the
> dependent variable and C as the independent variable. You can also compute
> the residuals for B (call it B-B') using B as the dependent variable and C
> as the independent variable.
>
> The definition of partial correlation between A and B adjusting for C is the
> correlations between (A-A') and (B-B'). The definition of part correlation
> between A and B adjusting for C is the correlation between (A-A') and B.
>
> The instructor claims that the part correlation is usually better (more
> interpretable?) but that SPSS and other software will not compute such a
> correlation.
/stats=zpp option in regression (or menu equivalent) gives
zero-order, part, and partial rs for each predictor.
> Does any of this make sense? Why would you ever want to use a part
> correlation?
I was surprised that some people strongly disagreed with the
observation that part was "usually better." Part^2 represents
portion of total variability in Y uniquely predicted by that X
(i.e., X residualized on other Xs), whereas partial^2 represents
portion of variability in Y not already explained by other
predictors that residual X explains. It seems to me that one
might often (not always, of course) want to know what proportion
of the total variability in Y is predicted by X-residual, rather
than what proportion of the unexplained variability in Y is
explained. The latter will depend on many things other than the
relation between that X and Y (e.g., the number and nature of the
other predictors in the equation).
Best wishes
Jim
============================================================================
James M. Clark (204) 786-9757
Department of Psychology (204) 774-4134 Fax
University of Winnipeg 4L05D
Winnipeg, Manitoba R3B 2E9 [EMAIL PROTECTED]
CANADA http://www.uwinnipeg.ca/~clark
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