Control Groups in Causal Inference
Social Science Statistics Blog: The Value of Control Groups in Causal Inference (and Breakfast Cereal): "The Value of Control Groups in Causal Inference (and Breakfast Cereal)
A few years ago, I taught the following lesson in my daughter's kindergarden class and my graduate methods class in the same week. It worked pretty well in both. Anyone who has a kid in kindergarten, some good graduate students, or both, might want to try this. It was especially fun for the instructor.
To start, I hold up some nails and ask 'does everyone likes to eat nails?' The kindergarten kids scream, 'Nooooooo.' The graduate students say 'No,' trying to look cool. I say I'm going to convince them otherwise.
I hand out a little magnet to everyone. I ask the class to figure out what it sticks to and what it doesn't stick to. After a few minutes running around the classroom, the kindergardners figure out that magnets stick to stuff with iron in it, and anything without iron in it doesn't stick. The graduate students sit there looking cool.
From behind the table, I pull out a box of Total Cereal (teaching is just like doing magic tricks, except that you get paid more as a magician). I show them the list of ingredients; 'iron, 100 percent' is on the list. I ask by a show of hands whether this is the same iron as in the nails. 3 of 23 kindergarten kids say 'yes'; 5 of 44 Harvard graduate students say 'yes' (almost the same percent in both classes!)."
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Perfect... seems so damn obvious...
No, the iron in total is NOT the same as in the nail. The nail contains elemental iron while the cereal contains ferrous iron (Fe2+). Iron is ferromagnetic while ferrous iron is ferrimagnetic.Both will be attracted to a magnet, but the Fe2+ is less attracted.
Finally, when you look at the actual iron content of Total, it is difficult to believe that there is enough iron in any given volume of cereal to yield significant attraction to a magnet.
I strongly suspect an alternative explanation. Perhaps there is a binder in the total, or rice and wheat have different electrostatic properties. One thing I will bet on, however, is that it was NOT the iron in the total that made it stick to the magnet!
Pulmon: +1 SCIENCE.
The article doesn't suggest that the magnets stick to the box of cereal.
Having eaten Total in a prior lifetime, it tastes like nails and DOES INDEED have big pieces of iron in it. May not be made from nails, but it is indeed magnetic....
In 10th grade chemistry our teacher did a little experiment. She dumped a box of Total into a big beaker and filled it with water. At the bottom of the beaker was a mechanical stirrer (a magnet that is induced to rotate).
The next day we came to class she turned off the stirrer and removed the magnet. It was covered in iron filings.
I am quite certain I remember an episode of "Mr. Wizard" where he ground up breakfast cereal and showed the results responding to a magnet.