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. If you live in the United States, there’s an 87% chance that you don’t share all three of these attributes with any other U.S. resident. Second, there may be particular data sources available (Sweeney used a Massachusetts voter registration database) that let people do searches to bootstrap what they know about someone in order to learn more — including traditional identifiers like name and address. In a very concrete sense, “anonymized” or “merely demographic” information about people may be neither.

Coaching moment: Think of how many grocery store, membership applications, and online accounts have your name, zip code, gender and birth date. Many of the contractual terms that we agree to when we apply for these services make reference to how the company plans to use their data. In some cases, they claim to use “aggregated data” which does not identify us by name. However, if we put a few of these databases together (you know this is happening, right?), there’s a lot of data available about us. Specifically.

Think about who is asking for your data, and what need they might have for it. I encourage you to think more critically about your data sharing practices. It might not be safe to think that anonymized data stays that way.

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