Lies, damned lies and statistics

A recent email from Karen Lawrence Öqvist mentioned the differences in how we measure statistics such as the odds of dying in a road crash or a plane crash. I had suggested in my book “Managing the Human Factor in Information Security” that you are more than a thousand times more likely to die in a road crash than an airplane. Is this a fair claim?

This subject is illuminating for any student of statistics, as there are huge differences in the conclusions drawn by different authorities. Some observers claim that air and car travel are equally risky. Others point to huge differences with factors ranging from ten times to two thousand times. Yet these claims are all based on analysis of independently compiled statistics. What accounts for such astounding differences?

There are many factors at work. The scope of the statistics used makes a big difference. Figures are generally compiled on an international basis for air crashes, and a national one for car accidents. One or two well well-referenced quotes mix the two. Some statistics also exclude small aircraft and military jets, which account for many reported accidents. Most figures include crew deaths as well as normal passengers. But not everyone travels regularly by air, so the risks vary significantly across different types of traveler. There are also marked differences between individual airlines and between countries. First-world airline accident statistics are lower, and road deaths in developing countries are much higher, as you would expect. 

The time of the statistics also makes a big difference. Air fatalities have been dropping rapidly, and are around half of the rate they were ten years ago. Road deaths are falling slowly in developed countries, but climbing steeply elsewhere. So comparisons vary greatly depending on the particular year and time period. Even more significant is the question of whether you compare just the total number of deaths, or the adjusted number of deaths per trip, per hour, or per mile. Such comparisons vary enormously, perhaps by an order of magnitude. 
 
So how did I arrive at my “more than a thousand” figure? In fact it was a simple calculation based on worldwide figures. There are estimated to be more than million road deaths each year and less than a thousand air deaths. No spin at all, though perhaps the absence of any adjustment represents a spin in itself. 

The real learning point is that you can put any or all of these factors together to achieve just about any result you might want. That’s welcome news for lobby groups campaigning for or against air travel, but confusing for ordinary travelers. So, in future, take all such claims with a pinch of salt.

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