Misleading head count of injuries

“Today, Helmets for All� [July 24] reports the number of head injuries per year for a wide variety of sports; as was clearly implied by the discussion of pole vaulting, many people will interpret these as direct representations of the risk of head injuries entailed by various activities. However, this is a false interpretation -- raw counts tell us nothing about the likelihood that an event will occur. Rather, the number of events must be related to the frequency with which individuals are exposed to the type of risk at issue.

For example, each year there are many, many more head injuries due to bicycling than due to pole vaulting, yet, in itself, this does not prove that bicycling is more dangerous than pole vaulting, since many more people ride bicycles than pole vault. Ideally, such counts are expressed as a function of the number of person-hours but, since such data are often hard to come by, we can settle for the number of people engaging in the activity -- even rough estimates of the number of participants help readers to contextualize the information presented.

DANIEL FESSLER

Associate professor of biological anthropology

UCLA

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I don’t know if helmets have made “us� safer, or more specifically, if they have made those who wear them safer. Your statistics are inconclusive, and statistics are a weak method of proof, anyway.

But I am sure that the helmet movement, like the seat belt movement, has made Americans less free. No adult should be required by government mandate to wear any helmet, and no child should face such a requirement from anyone other than his parents. That includes the state schools as government agencies. Or should we change the words “the land of the free� in the national anthem?

DAVE CLOSE

Costa Mesa