Hijacking risk measures

Statistics are dually known as useful and misleading. Another relevant saying is that a little knowledge is a dangerous thing.

3 Series
Creative Commons License photo credit: star5112

CAR magazine used to have a short section covering the number of complaints received from readers separated according to car brand. The problem with that sort of analysis is that it ignores the relative number of cars from each brand on the road. The “exposure” of toyotas to problems is much higher than maseratis since there are rather more toyotas on our roads. If CAR magazine received an equal number of complaints from drivers of maseratis and toyotas, it would suggest anything but an equal likelihood of having problems from each of those brands. I obviously wasn’t sufficiently convincing when I offered to help them devise a less biased measurement criterion.

This is an example of a common problem with random events – it is important to consider what could have happened as well as what did happen when understanding the results.

In a related example, iAfrica has an interesting article on car hijackings in South Africa. They include a list of the top ten hikacked cars. The list is interesting, but difficult to interpret without knowing how many of each vehicle were hijacking AND how many were on the road, able to be hijacked. If the Maserati Gran Turismo were on the list, I would be wary of driving one!

I’ve included the list below, with the number of sales added in brackets (from NAAMSA).  These are recent sales which is an imperfect proxy for the total volumes of vehicles on the road. For example, the

  • Toyota Hiace (discontinued, but 1,220 of the replacement Quantum/Ses’fikile were sold)
  • Nissan 1400 (only 11, but 412 for NP200 which is the replacement model for the now discontinued 1400 bakkie)
  • Toyota Hilux (1,583)
  • Toyota Venture (none, but I think the Avanza is the current replacement and 360 were sold)
  • Isuzu KB (896)
  • Toyota Corolla (1,206 including Verso and RunX)
  • BMW 3 Series (960)
  • Nissan Hardbody (444)
  • VW Citi Golf (1,709)
  • Mercedes C Class (1,457)
  • VW Polo (2,151)

What may not be clear from this is that the “top ten list of hijacked vehicles” pretty closely reflects the “top ten vehicles on the road” list. New models and sales trends obscure the picture as evidenced by the comments in parenthesis above. However, it might be clearer if I mentioned that only the Opel Corsa Utility (1,278), Ford Fiesta (500 sales), Ford Bantam (496), VG Golf/Jetta (453), Ford Ranger (376) , Audi A4 (317) and Audi A3 (242) had more than around 250 sales and weren’t on the top ten hijack list.

Ford seems to fare rather well given moderate sales and low hijack propensity. I don’t know whether the Opel Corsa Utility and Ford Bantam have picked up some of the sales from the Nissan 1400 bakkie which again may skew the results.

The conclusion of this analysis is that you are not necessarily more at risk from driving any of the vehicles on the top ten hijack list, because you are one out of many driving those same vehicles. If I had exact numbers of hijackings by brand, and an accurate list of registered vehicles I could perform a more accurate analysis which might reveal some truly high risk vehicles.

Out of interest, there were four Maserati Gran Turismos sold in January 2009. Given the difficulty of selling a stolen Maserati, you might argue that it would be safer from hijacking than a ubiquitous citigolf or corolla. I have other, less sensible reasons for wanting one.