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Jeff Corey
22nd January 2004, 02:26 PM
A Demonstration of Charpentier’s Illusion.
In 1891, Charpentier published the first systematic report of what has been called the Size-Weight or Charpentier’s illusion. A classroom demonstration of this illusion may be constructed with a large container, such as an empty 9.5 L water container, 13 film cans, a Kg of lead shot and a scale.

The spout is removed from the water container to reduce weight (it should weigh about 160 to 200 g) and the film cans are filled with the lead shot to provide a series of stimuli to allow people to attempt to match the weight of the water container.
A good series of matching weights would be (in grams): 5 (trim the bottom off), 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100, 120, 140, 180 (or whatever the water container weighs), 220, and 260. Label the film cans.

To perform the demonstration, first keep the scale hidden. Line the film cans up in ascending order on a desk with the water container behind them. Instruct the students that they are to lift the cans and cannister and report which of the film cans appears to weigh the same as the large cannister.

I do this individually, with students coming into the classroom one at a time to eliminate an audience or conformity effect. After the data are collected (usually by the first student to make a judgement, to keep things honest) construct a frequency distribution on the board. Get them to calculate the mean, median and mode. For more advanced students, ask them to do a statistical test and write up the lab.

When the swiftest student asks, “How do we know those are the real weights?”, whip out the scale and ask her to perform the measurements.
The point is that 129 out of 155 skeptics at TAM2 picked the 6oz water container to weigh less than a 3 oz lead sinker that was about 1250 times as dense as the water container. This stresses the importance of objective measurement rather than subjective judgement in critical thinking and science.

Jeff Corey
Psychology Department
C. W. Post College

cbish
23rd January 2004, 12:01 PM
Cool! Thanks Jeff. Mind if I print it out?

cbish
Science god

Jeff Corey
23rd January 2004, 01:03 PM
Not at all!
I handed it out at TAM2 to any teacher or anyone else who wanted it.
I would like people to use it in teaching, science fairs or just for the fun of it.
Enjoy.

Boo
23rd January 2004, 10:47 PM
Most awesome. Passed it onto a friend that teaches middle school (8th grade) science. I am planning on working it into my classes for Emergency Medical Technician for the fall semester.

Now I really wish I had been able to go to TAM2.

Boo

T'ai Chi
24th January 2004, 12:11 AM
Excellent Jeff. :)

In an undergraduate statistics lab I taught last year (as a grad. student teaching assistant) I did an activity where people just reported their weights and then made a dotplot and calculated some descriptive statistics based on those, and then actually weighed themselves and made a dotplot and calculated some descriptive statistics based on those.

Of course people discovered that their subjective measures tended to be in increments of 5, usually way too low, and other oddities, and that in some classes the inferences drawn from some standard statistical tests were different depending on whether the subjective or objective numbers were used.

I actually encountered this same thing (but with Auklet counts) in a more real life setting during my duties as a grad. student statistical consultant. A group of zoology students I was consulting with for their Ph.D theses used subjective counts and actual thorough counts of the bird populations and were confused because they drew different inferences depending on which numbers they used. :)

kittynh
25th January 2004, 05:39 AM
all I can say Jeff is you must rock as a teacher.

I did your weight thing with the preschool. They loved it. I had a scale they could use so later they could weigh the two objects for themselves. One of my three year olds said, "ooohhh, nature is tricky!"

Phil Plaitt gave me the best Hubble pics which I put up all over the classroom. the kids are drawing their own "Hubble" drawings now.

Thanks for the great lesson!

Jeff Corey
25th January 2004, 05:54 AM
Kitty,
As an artist, you might enjoy this one from psychologist Richard Gregory.
Get a cheap plastic mask of a normal human face and trim the sides down to eliminate the extreme borders (forehead and chin, especially). Mount it on a vertical piece of posterboard with the nose to the board so people view the inside of the mask. Illuminate the mask from below so that the normal light and shadow cues are reversed.
If people view this with one eye, the face will appear to move with them as they walk past it.
This is because the normal cues for depth are reversed and motion parallax produces the paradoxical effect.
You can also construct a cube that does the same thing. My paper "Constructing the moving cube illusion" was published in Activities Handbook for the Teaching of Psychology, vol.4 in 2000.
If you google for "moving cube illusion", some sites have my instructions and diagrams on them.

Jeff Corey
27th January 2004, 06:49 PM
Charpentier Illusion Update:
I did the same demo in class today as at TAM2. There was the empty plastic water can (9.6 L, 176 g) vs the 7.5 mL, 88 g lead sinker. The question was, "Which one weighs more?"
All 13 picked the sinker, at half the weight. Binomial Test, p = .5 to the 13th.
And double blind controls were in effect.

Jeff Corey
3rd February 2004, 06:19 AM
And again. In a Critical thinking class of 17 people, 4 correctly judge the larger container to be heavier.
The significance was p=.0245 (one tailed Binomial Test).
Did anyone else try it?