## 04 July 2007

### what good is an "average"?

Ugly Airline Math: Planes Late, Fliers Even Later -- from the July 5 New York Times.

Basically, airlines count the "average delay" by how late the average plane is. But what really matters is how late the average passenger is, which is a different problem because of connecting flights. If you're scheduled to have a ninety-minute layover and your incoming flight is forty minutes late, then suddenly your connection is tighter but you'll be okay, and moreover you get to your destination at the same time you would have otherwise. But if your incoming flight is two hours late, then you have to get on the next flight -- which means you'll be far more than two hours delayed.

The average delay for planes is 15 minutes; for passengers, 25.

But is this statistic even meaningful? The article talks of people who were, say, 12 or 24 hours late to their destinations. The mean isn't meaningful for these sorts of distributions, which are very skewed. Sure, it's easy to calculate. But if I were a passenger, and I had to pick one number to know about an airline's performance, it wouldn't be the mean delay. I'd want to know what percentage of their passengers get where they're going more than, say, an hour late. Or which percentage get there not on the flight they were originally ticketed on. (The same flight number on the next day doesn't count as the "same flight", although I bet they'd find a way to make it count.)

But whatever you do, the airlines will probably find a way to game the system so that they look good under the performance metric in question without actually satisfying their customers. This is because people will demand some simple metric they can understand, even though I suspect that any single number that measures an airline's performance would be something like, say, Google's PageRank -- impossible for the layperson to calculate but still remarkably useful.

#### 1 comment:

frank said...

There cannot be a single number for passenger delays. Lets assume I have a 1 hour lay-over, there is no lag time between getting off one plane and onto another, and my second flight is always on time. If the first flight is 59 minutes late, then I make the flight and I'm on time. If the first flight is 61 minutes late, I am delayed by hours.

What bothers me even more is that planes schedule arrival times much later than the expected flight time. This appears to lower the delay time by making a 30 minute delay appear to be 0. In fact, I'm waiting for them to tack on another 15 minutes to every arrival time, then they can report that their average delay is 0.