When is good news not good news? When you can't trust the data.

If you've been flying a lot lately, you may (a) be thankful, because this means you still have a job, and (b) be surprised to learn that the latest government statistics show that airlines' on-time arrivals rate has improved.

According to Susan Stellin's piece in yesterday's New York Times, airlines ran on-time 79% of the time in April, up from a relatively steady 75% rate. This sounds like good news, but let's delve a little deeper, shall we?

It turns out that not every airline is required to report its flight-delays data, but only those airlines with more than 1% of domestic passenger revenue. That, according to Stellin, "leaves out roughly 25 percent of all domestic flights."

Among carriers not included are many regional partners of the larger airlines, and of course those smaller partners are sometimes the only airlines serving small cities. Nor are international carriers included.

While the number of airlines submitting information to the Transportation Department is up (19 carriers this year, up from 10 in 2002), this is still not a complete census from which travelers could get a really accurate picture. Nor is it a statistically balanced sample. It's just lousy data.

My mantra here is, "Transparency, transparency, transparency." We don't just need more data, we need better data. Airlines should report voluntarily (as, according to Stellin, Pinnacle Airlines does), but if they won't -- call me cynical, but I suspect they won't -- they should be required to.