Wednesday, July 09, 2008

How many laptops are lost - lies, damn lies, and statistics

A Ponemon Institute survey (sponsored by Dell) says that about 12,000 laptops are lost at US airports every week. But when Computerworld magazine called some of the airports cited in the study, the numbers they gave differed dramatically from the study reports. For example, in Miami Ponemon said 1000 laptops/week while TSA said that there were 68 stolen and 480 turned in - for all of 2007. At Washington National airport (*) Ponemon claims 450/week, but TSA says 276 laptops were turned in for the whole year.

So what's the truth here? Is TSA underestimating, or is Ponemon exaggerating for effect? As usual, the truth is probably somewhere in between. But if I had to make a bet, I'd guess it's a lot closer to the TSA's numbers than Ponemon's.

Let's try some round numbers. Miami airport had 33 million passengers in 2007. (I don't know if that includes people changing planes, and whether that includes both departures and arrivals.) But let's assume that's only departing passengers, which comes out to about 650,000/week. Let's assume that half of the lost laptops are at security checkpoints, since that's the place where things tend to get misplaced the most. So let's take half of Ponemon's numbers, or 500/week. In very round numbers, that means 1 out of 1000 passengers loses their laptop going through security. So if you're flying on a 747 (which seats about 400 people), the odds are roughly 50% that someone on that flight lost their laptop on the way to the plane.

I find that hard to believe - you'd think that if it happened that frequently, there would be paniced people running around airports on a regular basis looking for their laptops... and we'd all hear horror stories from our friends and relatives.

This is all back of the envelope calculation, so even if I'm off on some of these numbers, it's not going to change the overall answer.

Back of the envelope calculation is a useful technique to sniff out the unlikely in statistics. I'm surprised that Ponemon didn't ask "do these numbers really seem likely".

(*) Out-of-towners call it "Reagan", but to locals, it's always National.


Post a Comment

Subscribe to Post Comments [Atom]

<< Home