HEALTH ISSUES

Lies, Damned Lies, and Statistics

I overheard an interesting conversation between two young women as I was waiting in line to ride The Comet, the granddaddy of roller coasters at Great Escape Fun Park in upper New York State. One was preparing to study in Australia and was describing her travel plans. Her friend thought Australia would be exciting but added that she would never go herself because flying was too dangerous. The prospective traveler responded that she wasn't concerned about a plane crash but was worried about the risk of developing deep vein thrombosis—a potentially fatal blood clot—during the trip. As soon as I heard this comment I knew that she must have watched the same TV talk show I had the day before.

While the specifics of this conversation might have been unique, the gist of it was not. Details aside, the young women were involved in risk evaluation, something we all do on a regular basis. Just think about how often we ask ourselves whether or not we should be worried about mercury in tuna, radiation from cell phones and microwave ovens, aspartame in diet drinks, and the reported link between estrogen supplements and an increased risk of cancer. Life often comes down to analyzing risks. But most people do not realize how difficult it is to perform this analysis in a meaningful way.

Let's start with something easy, like the risks of air travel. Flying is actually remarkably safe. Since the advent of commercial air transport around 1914, some 15,000 people have perished in airplane crashes. In North America alone more than three times that many people die in automobile accidents every year! You are far more likely to arrive at your destination if you fly than if you drive. Unfortunately, traffic on the highways increased significantly after September 11, 2001, resulting in many deaths that would not have occurred if people had flown. So why are people so scared of flying? Because they don't think statistically—they think emotionally. People have the perception that their destiny is in their own hands if they are driving a car; they feel that they have relinquished this control when flying on an airplane. Also, there is a greater likelihood of surviving a car crash than a plane crash, which is another factor that weighs on people's minds. But the statistics show that over a lifetime, you are 100 times more likely to die in a car accident than in a plane crash. Basically, you are more likely to be struck by lightning or win the lottery than die in an air disaster. Flying to Australia is safer than driving to the Great Escape from New York City, which is what the two young women had done.

Now, about the deep vein thrombosis. The TV show I had seen focused on what has come to be called "economy-class syndrome" and began by recounting the tragic case of a healthy British woman in her late twenties who collapsed at Heathrow Airport in London after a long flight from Australia. She died within hours from a blood clot in her lung that had originally formed in her leg while she sat in a cramped position for an extended period. The show also included interviews with physicians in Hawaii who described similar incidents. There is no question that such deep vein thrombosis can occur, but the number of people who develop this condition is very small when compared with the number of passengers that fly. Indeed, a study reported in the New England Journal of Medicine found no association between air travel and deep vein thrombosis. On long flights, passengers should certainly be encouraged to move around, particularly if they are seniors, have a history of heart disease, are pregnant, or are taking estrogen supplements. The risk of deep vein thrombosis on a flight is a minute statistical blip, but if you take three victims and put them on a talk show together, viewers will think that the air travel industry is in midst of an epidemic.

If you want something to worry about on a flight, worry about the air quality. Although the recycled cabin air is filtered and mixed with fresh air, during flight many passengers complain of nausea and flu-like symptoms, which are consistent with a reduced oxygen supply. Of even greater concern is the spread of infectious organisms within the confines of an airplane cabin. In one documented case, passengers were kept on board while a plane underwent a minor repair. One of the passengers had a case of influenza A, which spread to three-quarters of the other travelers.

The way data are communicated can also affect people's perception of risk and the decisions they make. Take, for example, the recent study that showed a 30 percent increase in risk of breast cancer among women taking estrogen supplements. Sounds terribly frightening! But consider that within a ten-year period, about 3 to 4 percent of menopausal women will be struck by breast cancer. A 30 percent increase in this risk means that that number rises to 5 percent; suddenly the 30 percent increase in risk doesn't seem quite as impactive. Putting it another way, a postmenopausal woman who takes estrogen reduces her chance of remaining cancer-free from about 96 percent to 95 percent.

Consider also this example: A pharmaceutical company's ad, aimed at physicians, touted a drug's ability to reduce the risk of heart attack in a group of patients by 25 percent. In the wake of the ad, the number of prescriptions being written shot up. In reality, however, the drug study showed that 0.4 percent of patients had fatal heart attacks when not taking the drug as opposed to 0.3 percent of those who did take it. When statisticians examined the data they concluded that if more than seventy patients were treated with the drug for five years, the treatment would help just one of them avoid a heart attack. With results stated in this way, the drug sounds far less appealing.

And how about this: Benzopyrene in charcoal-broiled foods is a known carcinogen, because in large doses it will cause cancer in rodents. Based on the animal model, scientists estimate that eating 100 charcoal-broiled steaks will increase the risk of cancer by 1 in 1 million, a number that has arbitrarily been selected as a "red flag." What does this statistic really mean? Since the risk of cancer over a lifetime is about 1 in 3, eating these steaks will raise that risk by 0.0003 percent. It makes more sense to limit consumption of steaks because of their fat content, not because "they cause cancer."

It would have been fun to engage the two young women at the Great Escape in a discussion of risks. I could have told them about the sixty-four-year-old man who developed "shaken baby syndrome" after riding a roller coaster. Given that the two were smoking like steam engines, I also would have liked to mention that every 1.4 cigarettes increased their risk of cancer by 1 in 1 million. But with all that secondhand smoke around, I didn't want to venture any closer. Risk analysis is a risky business.

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