Letters

Thinking probabilistically
In the excellent article “Beware the long tail” (SN: 11/5/11, p. 22), the areas under each curve in the figure “Spotting the tail” should be unity (the total probability must be one). Therefore, the red curve should be lower in the center than the black one.
Filson Glanz, Durham, N.H.

Yes, the area under each curve should add up to one. In this case the graphic was shown without a y-axis to illustrate the basic point about the shapes of such curves; it would have been more appropriate to have shown the curves separately rather than superimposed. —Rachel Ehrenberg

The graphic “Event prediction” shows the Shanghai Stock Exchange Composite Index starting to decline at the beginning of the predicted period (gray bar) in 2009, but the text states that the bubble burst July 29, two days after the prediction of July 17–27. What is the explanation?
Bobby Baum, Bethesda, Md.

The graph was reproduced from a paper in the Journal of Economic Behavior and Organization, which misplaced the gray bar for the 2009 prediction. The bar should have been farther to the left, such that the gray area ends right before the index’s fall.
— Rachel Ehrenberg

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Standardizing the boom
I read your article “Explosive goes boom, not too soon” (SN: 10/22/11, p. 10) and had to wonder, why 2,940 grams for the drop test weight? It certainly doesn’t convert to a straight­forward number of pounds or ounces.
Paul Johnston, Kamuela, Hawaii

Materials chemist Adam Matzger of the University of Michigan in Ann Arbor replies: “This is the weight of the 3.5-inch steel drop weight we used. It is not a standard size; in fact, there is not really a standard, so we calibrated our drop weight system against established explosives.”

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Greedy monkeys
The research that concludes kids are generous and chimps are not (“Kids share, chimps stash,” SN: 8/27/11, p. 10) based on two dissimilar studies makes no sense to me. The children were sharing marbles, the chimps food. Ask a bunch of hungry 3-year-olds to share chocolate chip cookies and see what happens. Then, if they do share, I might believe the children are altruistic and the chimps are opportunistic.
Eleanor K. Summer, Gainesville, Fla.