[from Gary Cziko 930429.1730 UTC]
[Note. UTC is the now accepted "scientific" term for what used to be GMT
(Greenich Mean Time). It stands for Coordinated Universal Time with the
letters moved around a bit either to make some French people happy or to
avoid making it look like marching music.]
I have a rubberband demo that makes an informative contrast between the
IV-DV approach to behavioral research and the CV (controlled variable)
approach.
Use two knotted rubberbands but tie to the end of one of them a string at
least about 10 cm long (another rubberband is OK, but a string works
better).
Now, ask someone (the subject, S) to take the end of one rubberband and
keep the knot over a point as the Experimenter (E) holds the end of the
other rubberband (keeping the string hanging down or nestled in his hand)
and introduces slow disturbances.
You will, of course, find a regular relationship between S's hand position
and E's hand position. The E's hand appears to be the IV and the S's hand
the DV. You can quantify this relationship and make all kinds of accurate
predictions concerning what S will do when E does something. In fact, I
suspect that such predictions using E's hand position as the IV will allow
you to make (under the current arrangement) BETTER predictions concerning
what S will do than worrying about the CV (indeed, if control is good,
watching the CV is a pretty lousy way of making predictions since it will
not move much (probably because there is no info about the disturbance in
the perception, blah blah blah).
So what's wrong with this way of doing research and making predictions? I
suppose nothing if all you are interested in is making predictions and the
arrangement does not change. But watch what happens if E now keeps his
hand still and pulls on the string with his other hand. Now the original
IV does not change at all BUT THE S'S DOES! Or have the E hold the string
steady while he gently slides (without tugging) his previously IV hand up
and down the rubberband, and see that THE S DOESN'T RESPOND AT ALL. What
has happened to the nice relationship between IV and DV? It no longer
seems valid. But if we had kept our eye on the CV, we would have had no
trouble making predictions about what the S would do if E pulled on the
string or held the string steady and slid up and down the rubber band with
the previously IV hand.
I would like to propose that the IV-DV relationship is what Dag Forssell
would call a descriptive theory while the CV theory is a generative theory.
I suppose that in some cases an IV-DV theory of behavior can be useful and
predictive, but it fails to be so if the relationship between the IV and CV
is itself disturbed. And I think that the world we live in is one in which
the IV-CV relationships are often disturbed and varying (this seems to be a
take-off on Bourbon and Powers's "Possible Worlds" paper).
To use another example, you may find a nice relationship between wind
velocity from the north and driver steering behavior on a straight road
running east-west. But aside from the fact that wind velocity cannot be
seen by the driver, this IV-DV will not tell you anything about what the
driver will do if the wind remains constant but the road itself starts to
tilt one way or the other. Knowing what the CV is (and a bit of physics)
will allow you to make correct predictions of the driver's steering
behavior. So it appears to me that one reason CV theories are better than
IV-DV theories is because the former are more useful (except, of course, in
the imaginary world where IV-CV relationships never change).
Note that IV-DV theories may suggest to a PCTer what the CV is. The
problem is, most IV-DV theories are based on very unreliable relationships
"found" over groups of individuals--but that's a whole 'nother story.--Gary
Cziko
ยทยทยท
------------------------------------------------------------------
Gary Cziko Telephone: 217-333-8527
Educational Psychology FAX: 217-244-7620
University of Illinois E-mail: g-cziko@uiuc.edu
1310 S. Sixth Street Radio: N9MJZ
210 Education Building
Champaign, Illinois 61820-6990
USA
------------------------------------------------------------------