Only 6% of crashes in New Zealand involve foreign drivers, according to the latest figures provided by the Ministry of Transport.
But in some remote regions of the South Island particularly popular with tourists for their scenery... foreign drivers are involved in about a quarter of all crashes.
These sentences come from a CNN article about a vigilante movement in those regions popular with tourists. The vigilantes snatch car keys from tourists who annoy them by holding up traffic along the scenic routes.
My friend Tonny saw this article and thought about Numbers Rule Your World. I love to hear stories about how you're able to relate the stories in my book to other real-world situations.
The 6% aggregate number hides the effect of tourists on the rate of accidents. The effect of tourists is different depending on which region one is looking at. For this particular article, only the 25% number is relevant--and even this point is not clearcut. I'd like to know whether the vigilante incidents are exclusively occurring in those "remote regions of the South Island", as implied.
The 25% figure does not address the more important question of whether locals or tourists are more likely to get into accidents in those regions. While the tourists accounted for one out of four accidents, they also comprise a large proportion of the traffic. If, say, 50% of the cars on those roads are driven by tourists, then they are disproportionately less likely to be involved in accidents. The base rate of tourist traffic is the missing data.
Further, a margin of error is useful, especially if few accidents occur in those remote areas.
Chapter 1 of Numbers Rule Your World deals with the notion of the statistical average and Chapter 3 investigates when it is appropriate to aggregate data and when it's not. Learn more about the book here.