Broken Promises of Privacy Failure of Anonymization

Gender, ZIP code, and birth date feel anonymous, but they aren't.

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.

Source:
http://www.eff.org/deeplinks/2009/09/what-information-personally-identifiable