[Phys-L] Re: Reaction Time (was Re: Human Error?)

[John Denker <jsd@AV8N.COM>]

On 05/02/05 12:12, Edmiston, Mike wrote in part:

> Rick says the pendulum spends more time
> at the ends of the swings and that makes it easier.  I say that makes it
> harder.

Yup.

>  It's fine to
> actuate the timer 50 ms early if we also stop the timer 50 ms early.  A
> person who consistently anticipates 50 ms early can be a better timer
> for anticipated events than one who is randomly off by 20 ms.

Amen, brother.  That's a reeeally important point.

To say the same thing in other words:  It's not the
"reaction time" per se that matters ... it's the variation
in reaction time, which is vastly less.

On 05/02/05 12:06, Frohne, Vickie wrote in part:
> ... the better way to handle the pendulum is to start the timer,
> count out 20 or more swings, then stop the timer.

That's the right idea.  Taking it one step further,
you can take 20 splits along the way (or if the events
are too close together, measure every Nth swing so you
get 20/N splits).  AFAIK every stopwatch sold these days
has a convenient split-time function.  Fit the data to
a straight line and you should get an uncertainty two
orders of magnitude less than the textbook "reaction
time".

All the above leads us to the larger point that
experimental uncertainty is not just a function of
the instrument, but also a function of how it is used.

===========================

Returning to the "human error" thread:  There is such
a thing as human error.  It is one contribution to the
overall uncertainty of the result.  It is neither
entirely the same nor entirely different from the
statistical uncertainty of measurement.  For example,
just last week I mistyped 115 when it should have been
1115.  That sort of error is not particularly uncommon,
and is very hard to treat using ordinary statistical
techniques.

Designing a good experiment is hard.

====================

Students need to realize that just because something is
_true_ doesn't make it _good_.

Suppose my student "measures" g = 200m/s^2 and prepares
a report saying

    1) This result is absurd.
    2) This must be due to human error.

Both of those statements are entirely true.

But so is the following statement:

    3) If you turn in that report as is, you're going
     to get a whole lot less than full credit.

Statement (3) is entirely true.  You don't have to
like it, but it's true.

Science isn't _just_ about finding the truth.  Often
we have the power to change things so as to obtain
another result that is just as true but much more
useful.

You want a result that is _true and useful_.  You
mustn't sacrifice one to get the other.

On 04/29/05 22:18, regarding "human error" Hugh Haskell wrote:
> I personally think that any student using that tired phrase should be
> lined up against the wall and shot, or a least thoroughly flayed with
> a cat-o-nine-tails.

I vehemently disagree with that, even after allowing
for an element of hyperbole.

I think there needs to be a sliding scale.  In order from
better to worse, a non-exhaustive list of points on
the scale is:
  *) Never making any mistakes.  (Nice work if you
     can get it, but hardly a realistic game plan.)
  *) Making a mistake, noticing, and fixing it.
  *) Making a mistake, noticing, and passing up
     several opportunities to fix it.  (Lame
     excuses and vague quasi-explanations such
     as "human error" or "something went wrong"
     fall in or near this category.)
  *) Making a mistake, and not even noticing that
     the result is absurd.
  *) Not doing the work at all.
  *) Cheating.

So when intro-level students cite "human error" IMHO there
should be partial credit for at least _noticing_ that the
result is absurd.  That's waaay better than not noticing.

Of course there remains a penalty for passing up all those
opportunities to fix it.

Also there is a penalty for giving a vague catch-all
explanation (e.g. "human error") when a more specific
explanation is available (e.g. "I measured the timing
at the top of the swing, when I should have measured
at the midpoint-crossing).

==========

As an illustration of the importance of a sliding
scale:  in _The Cartoon History of the Universe II_
Larry Gonick wrote:

   Chen Sheh:  What's the penalty for being late?
   Colleague:  Death.
   Chen Sheh:  What's the penalty for rebellion?
   Colleague:  Death.
   Chen Sheh:  Well, guess what?  We're late.

They rebelled, and Chen Sheh became the new king.
(op. cit., page 119)

Every time I recall that little dialog, it cracks me up.
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