Tuesday, 30 January 2018

Boy racers can't do statistics

Matthew Hansen wrote in the New Zealand Herald today:
Let's get this out of the way early - "boy racer" is a ridiculous and outdated term. Much of the country's modified car culture is propped up by the middle-aged, and by women. We're a world leader for female involvement in motorsport.
The "boy" aspect isn't exactly prevalent in the New Zealand Transport Agency's numbers for road deaths either. In the past 12 months, 379 drivers have been killed, and the three biggest age groups represented are those from 25-39 (103), 60-plus (91), and 40-59 (91). By contrast, deaths for those aged 15-19 number 25, and 52 for 20- to 24-year-olds.
I'm no rocket scientist, but those numbers are smaller. So why empower an outdated, incorrect term like "boy racer"?
Yes, those numbers are smaller, but that's often what happens when you compare the numbers of events happening to people in a five-year age group (15-19 or 20-24 years) with the comparable numbers for a 15-year age group (25-39), a 20-year age group (40-59), or a 40+-year age group (60 years and over). Before comparing the number of road deaths between age groups, you need to adjust for the relative number of people in each age group, to work out the incidence of road deaths. [*]

The number of road deaths this year so far are available from the NZTA road toll website. There are some small differences with the numbers that Hansen uses (probably because the statistics reported there are for the 12 months that end on the day you access the website), so for comparability I will use Hansen's numbers. The numbers of people in each age group are available from Statistics New Zealand (NZ.Stat) for 30 June 2017 (which is close enough to the mid-point for the year ended on some day in January).

While there have been 103 road deaths among people aged 25-39, there are 962,550 people in that age group. That works out to 1.07 road deaths per 10,000 people. Compare that with 52 road deaths and 355,830 people aged in the 20-24 age group, which works out at 1.46 road deaths per 10,000 people. For completeness, the other values are in the table below.

Clearly, the highest risk group is the 20-24 year age group when it comes to road deaths. I'm no rocket scientist either, but the numbers for other groups are smaller. In some cases close to half of the incidence for the 20-24 year age group. Maybe boy racers should stick to cars, not statistics? And the Herald should send its reporters to a course on some basic statistical literacy.


[*] Even better would be to adjust for the number of vehicle road miles travelled by members of that age group, which would be a 'more correct' measure of risk exposure (for vehicle travellers, so probably pedestrian and cyclist road deaths should be excluded first too).

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