What I love about the blogosphere is its collective intelligence — notwithstanding all the fuss about incivility etc. I’ve been fuming quietly ever since I read about the experience of Professor Walter Murphy, a decorated former marine who found himself on Homeland Security’s No-Fly list.

But now Ed Felten casts a calmer (and more informed) eye on the matter, and comes to a different conclusion:

There are two aspects to the no-fly list, one that puts names on the list and another that checks airline reservations against the list. The two parts are almost entirely separate.

Names are put on the list through a secret process; about all we know is that names are added by intelligence and/or law enforcement agencies. We know the official standard for adding a name requires that the person be a sufficiently serious threat to aviation security, but we don’t know what processes, if any, are used to ensure that this standard is followed. In short, nobody outside the intelligence community knows much about how names get on the list.

The airlines check their customers’ reservations against the list, and they deal with customers who are “hits”. Most hits are false positives (innocent people who trigger mistaken hits), who are allowed to fly after talking to an airline customer service agent. The airlines aren’t told why any particular name is on the list, nor do they have special knowledge about how names are added. An airline employee, such as the one who told Prof. Murphy that he might be on the list for political reasons, would have no special knowledge about how names get on the list. In short, the employee must have been speculating about why Prof. Murphy’s name triggered a hit.

It’s well known by now that the no-fly list has many false positives. Senator Ted Kennedy and Congressman John Lewis, among others, seem to trigger false positives. I know a man living in Princeton who triggers false positives every time he flies. Having many false positives is inevitable given that (1) the list is large, and (2) the matching algorithm requires only an approximate match (because flight reservations often have misspelled names). An ordinary false positive is by far the most likely explanation for Prof. Murphy’s experience.

Note, too, that Walter Murphy is a relatively common name, making it more likely that Prof. Murphy was being confused with somebody else. Lycos PeopleSearch finds 181 matches for Walter Murphy and 307 matches for W. Murphy in the U.S. And of course the name on the list could be somebody’s alias. Many false positive stories involve people with relatively common names.

Given all of this, the most likely story by far is that Prof. Murphy triggered an ordinary false positive in the no-fly system. These are very annoying to the affected person, and they happen much too often, but they aren’t targeted at particular people. We can’t entirely rule out the possibility that the name “Walter Murphy” was added to the no-fly list for political reasons, but it seems unlikely.