Retorts

Lowest Monthly Road Toll

New Zealand has just recorded its lowest monthly road toll in 30 years. Twenty-six deaths in road smashes for June 1995, and five fewer than the next lowest month, according to Alan Wilcox from the Land Transport Safety Authority.

This year's road toll of 268 (for 6 months) is one higher than this time last year.

I calculate six months' total of 268 deaths averages out at 44.6666667 deaths per month.

Taking 44 2/3 as the parameter (mu) of a Poisson discrete probability distribution, it would imply a 50.0% chance of 44 or less deaths in any particular month.

Actual deaths in June 1995 were 26, however. This number of deaths is expected probability 7.8608E-4, 0.08 %, which equals once in 1272 months, or 106 years.

Twenty-six or less deaths is a little less rare, of course, at approximately 1 in 564 months (47 years).

So there is maybe just a little something needing to explain why it happened already in 30 years? But I think not really. A "fixed" average of 44 2/3 deaths per month implies (assuming Poisson distribution) a return period of 47 years for not more than 26 deaths in one month.

So, if it just happened after 30 years (even 60 years) it wouldn't be that unusual.

The lay media like to report exceptional events. Such events, sometimes apparently rare, often may not have exceptional (non-random) causes.

Don S. McDonald, Wellington