July 27, 2014

# Air flight crash risk

David Spiegelhalter, Professor of the Public Understanding of Risk at Cambridge University, has looked at the chance of getting three fatal plane crashes in the same 8-day period, based on the average rate of fatal crashes over the past ten years.  He finds that if you look at all 8-day periods in ten years, three crashes is actually the most likely way for the worst week to turn out.

He does this with maths. It’s easier to do it by computer simulation: arrange the 91 crashes randomly among the 3650 days and count up the worst week. When I do this 10,000 times (which takes seconds). I get

The recent crashes were separate tragedies with independent causes — two different types of accident and one deliberate shooting — they aren’t related like, say, the fires in the first Boeing Dreamliners were. There’s no reason for the recent events should make you more worried about flying.

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 »

• Thomas Lumley

In case anyone wants the simulation code, there’s a version here

• Sammie Jia

I saw this from BBC as well. But is that rare that two incidences happened to one airline wihtin 6 months?

• Thomas Lumley

I don’t have the data to do an accurate calculation, but the birthday paradox suggests the probability isn’t very low.

There are lots of pairs of crashes within six months of each other, and there are about 250 IATA member airlines, so for any pair of crashes the chance that they involve the same airline is on the order of 1/250. Different numbers of flights per airline will drive the probability up, but if the biggest airlines are safer than will drive the probability down.