Re: [Phys-l] Independent Variables

[John Denker <jsd@av8n.com>]

On 10/31/2006 08:02 AM, Rick Tarara wrote:

> What is graphed on the x-axis and what on the y depends much more on what 
> information we really want and how much theory we might have going into the 
> experiment.  As mentioned, sometimes we 'know' what we want the slope to 
> tell us (say resistance in an Ohm's law experiment and therefore will plot 
> the measured current on the x-axis and the controlled voltage on the y).

Yes.

The part about "what we want" is particularly important;  it means that
questions of style and questions of taste are part of the game.  This
makes it particularly scary when there are standardized tests that
purport to know the Right Answer as to what is x and what is y.

As a specific example, consider a typical calorimetry experiment of
the sort that is routinely conducted in high school classes.

It is very likely that delta(E) [or delta(H)] is, to a decent
approximation, the "independent" variable i.e. the manipulated
variable.  During the subsequent data-analysis phase this can
be integrated to obtain the E [or H] variable.  Meanwhile let's
assume temperature is a dependent variable, i.e. an observed
(not manipulated) variable.

Now the twist is that it is conventional (certainly not necessary,
but conventional) to plot the results as H versus T, i.e. to plot
the more-or-less "independent" variable as a function of the
more-or-less "dependent" variable.

Therefore it cracks me up when I see lessons that purport to
teach the following, as if it were a general rule:

> > > The values of the independent variable are measured on the X axis and those of the dependent 
> > > variable are on the Y axis.

That's a quote from:
  http://www.bi.iup.edu/principles/laboratory/Graphing%20Data.htm


=========

   BTW I am not suggesting the "manipulated variable" is
   standard terminology, or that it deserves to become so.
   Experiment design is complicated enough that it deserves
   to be discussed in complete sentences, not reduced to
   rote memorization of jargon and slogans.