TSA Reveals: Alex Johnson is Spartacus

OK, that title's probably a bit obscure. I've been using "Spartacus" as a byword for issues of anonymity, after the scene in Spartacus where everybody claims to be Spartacus, so Spartacus could be anybody. For more details, see this post, or this one, or this one. More formally, the people that could have a particular identity form the anonymity set for that identity. The bigger the anonymity set, the more anonymous the identity.

The US Transportation Security Administration gives a good example of this principle in action, though they're pitching it toward encouraging people to be less anonymous to them. In case the link is broken when you read this, the ad in the upper-left corner shows a series of images with captions:

• Several people, with the caption "What do all these passengers have in common?"
• The same group, captioned "They are all named Alex Johnson"
• The three women in the group, captioned "Alex Johnson Female"
• Two of the women, captioned "Alexandria Johnson Female"
• One of the women, captioned "Alexandria Johnson Female 10/15/1967"

For more on just how few pieces of identifying information you need to narrow down to a small number of people, often to just one person, see this post.