Why do white sheep eat more than black sheep?
Why? Because there are more white sheep than black sheep.
The ACC understands this principle; their map of the ‘most dangerous regions’ for falls in NZ is based on number of claims per 1000 population in 2010. Even though Auckland has the most reported falls, it doesn’t have the highest risk.
It’s less clear that the Herald understands. They described a list from the NZ Transport Agency as New Zealand’s most dangerous intersections have been revealed for the first time. The list was actually the intersections with the greatest number of crashes leading to injury, not corrected in any way for traffic intensity.
Now, if you want to decide which intersections are the highest priority to redesign, the total number of serious crashes is a useful statistic. But if you want to know where it is most dangerous to drive, you need the denominators.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »