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31 October 2005

The Value of Control Groups in Causal Inference (and Breakfast Cereal)

Gary King

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!).

I show the students that the box is sealed (and I have nothing up my sleeves), Then, I open the box, spill some cereal on a cutting board, and smash it up into tiny pieces with a rolling pin. I take the pile of cereal around the room and let the kids put their magnet next to it and see whether the cereal sticks to the magnet. To everyone's amazement, it sticks!

Then I ask, are we now convinced that the iron in the nails is the same iron as in the cereal? All the kids in kindergarten and all the graduate students say "yes."

I respond by saying "but how do you know the cereal stuck to the magnet because it had iron in it? Maybe it was just sticky, like gum or tape." Now that I finally have their attention (not a minor matter with kindergartners), I get to explain to them what a control group is. And from behind the table, I pull out a box of Rice Krispies (which are made of nothing). We examine the side of the box to verify the lack of (much) iron, and then I smash up the Rice Krispies, and let them see if their magnet sticks. It doesn't stick!

Everyone gets to take home a cool fact (they love to eat the stuff in nails), I get to convey the point of the lesson in a way they won't forget (the central role of control groups in causal inference), and everyone gets a free magnet.

Posted by Gary King at October 31, 2005 2:18 AM