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.