NMA Article: Have You Been Misled By A Common Statistical Error?
When studying the causes of fatal accidents, the fatal accident rate (the number of fatal accidents on a per-vehicle-mile-driven basis) is the best available measure of highway safety. It is not subject to fluctuations caused by the chance number of people involved in any given traffic accident.
Unfortunately, the measure most commonly reported is the fatality rate, which is the number of people killed in automobile accidents on a per-vehicle-mile-driven basis.
A quick example:
- Two rural counties experience an average of one fatal accident per year
- Each county has one accident in which a deer crossed a road, causing the driver to swerve and hit a tree.
- In County A, a lone motorcyclist was killed.
- In County B, seven passengers on a bus were killed.
Assuming all drivers in both counties logged the same number of miles during the year:
The fatality rate would be seven times higher in County B, but the fatal accident rate would be identical.
If someone compared the two counties using the fatality rate, they could come to the conclusion that motorists are seven times more likely to die in a car-deer accident in County B. In reality, the odds of being in a fatal accident would be identical, because in each county there was one fatal accident with one common cause.
Because fatality rate statistics are more easily sensationalized, they are often the statistic quoted by the press.
From the perspective of the media, choosing between the fatality rate and the fatal accident rate can be the difference between a front-page story and something not worth printing.
Headline using fatal accident rate: "Motorists equally likely to be involved in fatal accident involving a deer in County A and B"
Headline using fatality rate: "Motorists in County B are seven times more likely to be killed by hitting a deer with their car"
The first headline would immediately be dismissed as not newsworthy.
The second headline would likely cause citizens in County B to be alarmed and demand new "safety" laws, lower speed limits, and more warning signs protecting them from deer on the roads. Politicians would then rush in to fix a problem that never existed.
You can hold the media in your area to a higher standard. If you see a news article quoting the fatality rate, write a letter or email to the paper and explain how misleading that statistic can be.
Image Credit: jonofpob
© 2008 NMA