Martin Taylor (2015.02.19,23,02)–
RM: You are still evading the question of how you would test to determine whether the position of a mass on a spring is a controlled. I can only assume it’s because you don’t believe that you can test to determine whether the position of the mass is controlled. Which is what I concluded in the first place; you don’t want to do research using the TCV because you don’t believe it can be done.
[From Rick Marken (2015.02.19.1550)]
RM: This is the test for the controlled
variable (TCV). Basically you are looking to see if the observed
effect of a disturbance to a hypothetical controlled variable is
less than expected from causal law. An understanding of causal
law is essential to being able to do the TCV.
That's the kind of thing I mean. Furthermore, the last sentence is
wrong even under your new definition of “causal law”. You are
saying that you couldn’t do the TCV on the spring mass unless you
could be sure that it was a standard spring that conformed exactly
to Hooke’s Law. If it was a fancy spring that got stiffer as it
was pulled more, you are saying that you couldn’t do the TCV. With
your criterion, how would you do the TCV to determine whether
someone was controlling the hardness of a frying egg?
before you conclude that x is under control
you do a step in the TCV that is rarely mentioned because it’s
usually obvious with a living control system: you look to see
whether the lack of effect is due to the action (output) of a
When did I say that it was unscientific? What I remember saying is
that scientific tests are ordiarily designed to compare one
hyothesis agains at least one competing possibility. At the time,
nobody had made any suggestion that the position of the mass on the
spring was under control.
I also, on another occasion, asked
Maybe that's why you say that I claimed the method you then
described, of seeing whether the mass on a spring moves according to
your presumed law of expected effect, was an unscientific way of
determining whether it was controlled.
You never answered my question, anyway. But that is normal, and to
As for your NEW question above, the TCV has several requirements, of
which you mention one right at the very end of you penultimate
I agree that this is too seldom mentioned. The other stages I think
of when I do a TCV are (1) Make sure that the system has a way to
sense the putative controlled variable (in this case the position of
the mass), and (2) check that the correlation between disturbance
and the environmental equivalent of the proposed controlled
perception is low. In this example case, check that the position of
the mass is poorly related to the applied force.
You didn't mention either of these required components of the TCV,
and you added something that is not usually considered a critical
part of it, a check whether the magnitude of the effect of the
disturbance is exactly what it would be in the absence of control.
That isn’t usually a part of the TCV because it assumes that you
have some independent way to determine what the magnitude of the
effect would be, such as an assumption you make in the case of the
spring, that you know exactly the Hooke’s Law constant of the
spring. If you do have such an estimate, then of course it’s a good
short cut, but usually you don’t.
So I ask again, assuming your criterion (that an understanding of
causal law is essential to being able to do the TCV), how would you
do the TCV to determine whether someone was controlling for the
hardness of a frying egg? (Or, if that’s too easy, whether someone
was controlling for the Government to be of a particular political
MT: ... How you turn this question into a claim
that I think the TCV cannot be done, even in that
trivial case, is quite beyond me…
RM: It's because you simply dodged my question about
how you would test to determine whether or not the
position of a mass on a spring is under control by saying
that it was unnecessary to do such a test in this case
because you already know it is not under control. But my
question clearly assumes that you don’t already know that
the position of the mass is not controlled. This would be
the case, for example, if you came across a mass on a
spring in a control engineering lab where it is possible
that the mass-spring system is connected to a control
system (in a way that you can’t see) that is controlling
the position of the mass. Your task is to figure out
whether or not the position of the mass is under control.
And the only means you have to test this is by applying
disturbances to the mass.
RM: My approach was to apply known forces to the mass
(by attaching known weights to the mass, say, assuming
the mass is suspended vertically from the spring) and see
if it had the predicted effect. The predicted effect is
given by Hooke’s law, which says that the result of
applying a disturbance force, Fa, to the mass is to move
it by an amount x = 1/k*Fa, where k is the spring
constant. If you know k then you need apply only one
disturbance force to see if you get the predicted
displacement, x. If you get the expected displacement then
the position of the mass, x,is not under control.
If you don’t get the expected displacement then it is
RM: But you don't really even need to know the spring
constant,k, to do this test. You just apply several
different forces to the mass and the prediction is that
the displacement of the mass, x, will be linearly related
to these forces, Fa, if the position of the mass, x, is not
under control. If the position of the mass is under
control, x will be approximately the same regardless of
the value of Fa. Of course, if the spring constant is
actually very large the disturbance forces (weights) will
have little effect on x even if the position of the mass
is not under control. So if you don’t know k and you get
very little effect of different values of Fa on x then,
before you conclude that x is under control you do a step
in the TCV that is rarely mentioned because it’s usually
obvious with a living control system: you look to see
whether the lack of effect is due to the action (output)
of a control system.
RM: This is the approach to the test that you deemed
unscientific. So I would like to know the scientific way
of determining whether or not a mass on a spring is under
MT: Usually when you do a test in
science you are comparing one hypothesis
against another…So why would you want to
do the TCV at all?
RM: No wonder you don't want to do the TCV; you
don’t think it can be done.
RM: ...could you please
explain the correct,
scientific way to determine
whether a mass on a spring
is under control.
Richard S. Marken, Ph.D.
Author of Doing Research on Purpose.
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