[From Bruce Abbott (951125.1510 EST)]
Bill Powers (951122.0830 MST) --
Now, consider the concept of an independent variable, a "cause." There
are, in fact, no independent variables -- that is, variables which
assume an arbitrary value with no relationship to anything else. When we
say "let the force rise to 1 newton, remain there for 0.01 sec, and
return to zero thereafter," we are postulating something that can happen
in only one way: some other variables must have changed in such a way as
to make the "independent" variable follow that pattern of values.
An "independent variable" is not a variable that "assumes an arbitrary value
with no relationship to anything else." It is a variable whose value
(state) is manipulated by the experimenter. Independent variables exist
whenever an experimenter manipulates a variable.
When independent variables are manipulated for experimental purposes,
either they change naturally as other environmental variables change, or
the variables are deliberately made to change according to a
predetermined pattern. The latter is almost always the case.
I am unclear about this distinction you make here. If an independent
variable is being manipulated for experimental purposes, it cannot be
changing naturally, as then one would not have an experiment but only a type
of correlational design. A weakness of purely correlational designs is
their inability to prove that the relationships they disclose are causal.
What this
means is that some human being must have conceived the pattern in
advance, and then either produced it by his own efforts or built a
machine that was adjusted until its output pattern matched the
predetermined pattern.
Yes.
The very concept of an experimentally-manipulated independent variable,
therefore, implies a control system in the background, a system which
can act to make a perceived pattern of change match a reference pattern.
Yes.
This makes the experimenter an integral part of the experiment. It is
the experimenter who picks which variable in a situation is to be
manipulated in a predetermined way and thus made into an independent
variable, and which other variable is to be observed and thus made into
a dependent variable.
Yes.
The experimenter determines what is to be cause
and what is to be effect.
This is only partly true. Let us say that I choose acceleration as the
independent variable and color as the dependent variable. So I accelerate
an object of a given mass at different rates and observe the object's color.
The color is observed to remain the same at all accelerations. It would
appear that acceleration (cause) does not influence the color of the object
(effect). That is, acceleration is not a cause of changes in color. Now,
you have said that I as the experimenter have chosen acceleration as the
cause and color-change as the effect, but my experiment discloses that
color-change is not an effect of acceleration. What can this mean? It
means that I as experimenter can choose what variable to manipulate as a
potential cause, and what variable to observe as a potential effect, but I
cannot determine (set) what IS cause, and what IS effect.
Now let us try a different experiment. I deliver a shock through the grid
floor of an operant chamber and the rat runs in a circle around the chamber
so long as the shock continues. Shock, the independent variable, appears to
be a cause of running, the dependent variable. Taking a cue from physics, I
then force the rat to run (perhaps by reinforcing running behavior through
successive approximation). To my surprise, the rat's running generates no
measurable current in the grid. The reason, of course, is that running and
shock-level are not mutually-influencing variables.
Since many different choices can be made, no
variable is inherently a cause and no other variable is inherently an
effect. Instead, the real system under study consists of a set of on-
going present-time relationships which, when properly described, remain
exactly the same no matter what manipulations are carried out.
But this does not appear to be the case in the rat experiment, does it? The
rat's action appears to be a function of shock-level, but shock-level does
not appear to be a function of the rat's action. Certain relationships
cannot be described as a set of variables in simultaneous equilibrium, in
which a forced change in any one variable is accompanied by instantaneous
changes in the others. When such mutual relationships do exist, one can
literally _define_ any one variable in terms of the others, as when E = I/R,
or I = ER, or R = I/E. But not all relationships involve mutual causality.
In an elementary control unit (ECU), such a simultaneous relationship exists
among p, e, o, and i. Application of the phrase "cause and effect" is
problematic for these members of the closed loop, which simultaneously
influence and are influenced by the other variables; perhaps "mutual
influence" would be better, if any such phrase is needed at all. An
experimenter attempting to identify the relationships among these variables
might begin by manipulating i and observing what changes take place in p, e,
and o, then choose e as the independent variable, etc. It would be
discovered that each variable influences the others.
Yet no manipulation of any of these variables will change either r or d,
although changing either of the latter will force a simultaneous change in
p, e, o, and i as they approach new equilibrium values. A set of simple
experiments designed to identify the causal relationships would show that
both r and d affect p, e, o, and i, that changes in r do not affect p, and
changes in p do not affect r.
The difference between reinforcement theory and either PCT or Newton's
laws is that reinforcement theory looks for cause-effect relationships,
while both PCT and Newton look for underlying relationships which remain
exactly the same no matter what events are occurring and no matter which
variables are chosen for manipulation.
I agree that reinforcement theorists have failed to properly characterize
the mutual influence of behavior on reinforcement and reinforcement on
behavior (within their system of terms), probably because most researchers
in experimental psychology have not received a proper grounding in the
necessary mathematics (for setting up and solving simultaneous differential
equations etc.). It is a deplorable state of affairs. However, I strongly
disagree that so-called "IV-DV" methods are at fault. As noted, one can use
such methods to identify which variable influence which, whether the
variables in question participate in an open-loop or closed-loop
relationship. We can agree that it would be preferable to refer to such
relationships as cases of simultaneous influence (either one-way or two-way)
rather than as cases of "cause and effect," as the latter implies a
time-order that may not be appropriate. However you refer to them, "IV-DV
methods" can reveal such influences, of either type.
In your vocabulary of present-time relationship, do you distinguish
relationships in which there exists a direct influence of one variable upon
another (or even mutual influences) from relationships in which two
variables change together over time, not because they directly influence
each other, but because each is responding to the influence of the same
third variable? In the traditional vocabulary one would say that the former
relationship is causal whereas the latter is only correlational. If you do
make this distinction, what terms convey it? It seems to me that the term
"causal" has continued to enjoy acceptance in psychology precisely because
it so nicely conveys this distinction. If we are going to replace the idea
of causality with this notion of simultaneous present-time relationship, do
we not still need to retain this distinction?
thinks that the only alternative to cause and effect is "independent-
variable, dependent variable" -- essentially the same thing.
I believe I have shown that they are NOT "essentially the same thing";
whether Skinner (and others) have treated them that way is another issue.
Regards,
Bruce