Research by Carnegie Mellon professor Latanya Sweeney and other experts shows that an alarming number of seemingly innocuous, neutral, or "common" data points, can potentially identify an individual online. "Privacy law, mainly clinging to a traditional intuitive notion of identifiability, has largely not kept up with the technical reality," says the EFF's Seth Schoen:
A recent paper by Paul Ohm, "Broken Promises of Privacy: Responding to the Surprising Failure of Anonymization", provides a thorough introduction and a useful perspective on this issue. Prof. Ohm's paper is important reading for anyone interested in personal privacy, because it shows how deanonymization results achieved by researchers like Latanya Sweeney and Arvind Narayanan seriously undermine traditional privacy assumptions. In particular, the binary distinction between "personally-identifiable information" and "non-personally-identifiable information" is increasingly difficult to sustain. Our intuition that certain information is "anonymous" is often wrong.
What information is "personally identifiable"? (EFF Deep Links)