Not That Sane. V Lakshman. Every Wednesday.

(F)lying statistics (Nov. 5, '97)

The host on a National Public Radio program repeated one of the lies for which statistics is roundly condemned. "It's safer to fly than to drive to the corner store," he mentioned in a throwaway comment. The fallacy in the NPR host's statement is that he is using the wrong measure. Is it safer to fly to Dallas than to drive there from Houston? No, it isn't.

Considering the number of trips made, it is much safer to drive than to fly. If you count your drive to the corner store as a trip, similar to flying a 10-mile distance, it is safer to drive there.

The statistic about flying being safer is gotten by dividing the number of accidents by the number of passenger-miles. The truer statistic is one that you get by dividing the number of accidents by the number of passenger-trips. You drive to the corner store about 150 times a year, travelling about 1500 miles. One airline roundtrip might involve 1500 miles. Are you really going to weight the two 1500 miles equally?

So, the next time, you are trying to decide whether to drive or to fly, forget the airline industry-inspired statistic. It is safer to drive there, if you can make it with a couple of halts.

Disclaimer: I am not really fond of flying. I hate the takeoffs and landings, the pain in the ears, the inane FAA-mandated safety instructions, the cramped aisles, the stewards with strained smiles, ... There is not much to love about flying, except that it gets you from here to there at about twice the cost of driving and a fraction of the time. Like most people in our jet-setting age, if I had to fly, I would choose a really long flight over a bunch of short hops. There is a rational explanation for such a desire -- after all, one less pair of landings and takeoffs means an additional margin of safety.


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