What does it take to be personally identifiable?

Last week California State Assembly member Michael Duvall (R-Orange County) was caught bragging to a colleague about having an extra-marital affair — next to a live mike. Along with his rather graphic descriptions he happened to mentioned his paramour’s age and birthday, and from this information OC Weekly was able to identify the woman:

“And so her birthday was Monday,” he said at the Wednesday, July 8 committee hearing. “I was 54 on June 14, so for a month, she was 19 years younger than me…”

According to voter-registration records reviewed by the Weekly, veteran Sacramento-based lobbyist Heidi DeJong Barsuglia turned 36 years old on Monday, July 6.

In this case there were other sources who also identified Ms. Barsuglia, and it’s not clear from the story whether OC Weekly actually arrived at her name through voter-registration records or simply used them for corroboration. However, EFF’s Deep Links reports that it’s actually not that hard to identify someone based on a few pieces of seemingly innocuous information like birthday, gender and zip-code:

Gender, ZIP code, and birth date feel anonymous, but Prof. Sweeney was able to identify Governor Weld through them for two reasons. First, each of these facts about an individual (or other kinds of facts we might not usually think of as identifying) independently narrows down the population, so much so that the combination of (gender, ZIP code, birthdate) was unique for about 87% of the U.S. population.

The linked-to abstract also mentions that about half the U.S. population are likely to be uniquely identifiable by only place, gender and date of birth, where place is basically the city, town or municipality where the person resides. And even if a search in a city as big as Sacramento came up with several potential matches, the hit that also happens to be a lobbyist working in an industry under Duvall’s committee would be easy to spot.